Deprecate `absl_deps` in templates and update documentation.

Bug: webrtc:341803749
Change-Id: I1ea6a2160c9a1d6d15a4f5e83f37d53f42215746
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/352200
Auto-Submit: Florent Castelli <orphis@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42382}
diff --git a/webrtc.gni b/webrtc.gni
index 860a69b..0e3487e 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -413,28 +413,24 @@
   configs = rtc_add_configs
   public_deps = rtc_common_public_deps  # no-presubmit-check TODO(webrtc:8603)
   suppressed_configs = []
-  absl_deps = []
 }
 
 set_defaults("rtc_library") {
   configs = rtc_add_configs
   public_deps = rtc_common_public_deps  # no-presubmit-check TODO(webrtc:8603)
   suppressed_configs = []
-  absl_deps = []
 }
 
 set_defaults("rtc_source_set") {
   configs = rtc_add_configs
   public_deps = rtc_common_public_deps  # no-presubmit-check TODO(webrtc:8603)
   suppressed_configs = []
-  absl_deps = []
 }
 
 set_defaults("rtc_static_library") {
   configs = rtc_add_configs
   public_deps = rtc_common_public_deps  # no-presubmit-check TODO(webrtc:8603)
   suppressed_configs = []
-  absl_deps = []
 }
 
 set_defaults("rtc_executable") {
@@ -562,20 +558,17 @@
       }
     }
 
-    # If absl_deps is [], no action is needed. If not [], then it needs to be
-    # converted to //third_party/abseil-cpp:absl when build_with_chromium=true
-    # otherwise it just needs to be added to deps.
-    if (absl_deps != []) {
-      if (!defined(deps)) {
-        deps = []
-      }
-      deps += absl_deps
-    }
+    assert(
+        !defined(absl_deps),
+        "`absl_deps` has been deprecated, add your Abseil dependencies to the `deps` variable.")
 
+    # Abseil dependencies need to be converted to //third_party/abseil-cpp:absl when build_with_chromium=true
     if (build_with_chromium && defined(deps)) {
-      absl_dependencies = filter_labels_include(deps, ["//third_party/abseil-cpp/*"])
+      absl_dependencies =
+          filter_labels_include(deps, [ "//third_party/abseil-cpp/*" ])
       if (absl_dependencies != []) {
-        filtered_deps = filter_labels_exclude(deps, ["//third_party/abseil-cpp/*"])
+        filtered_deps =
+            filter_labels_exclude(deps, [ "//third_party/abseil-cpp/*" ])
         deps = []
         deps = filtered_deps
         deps += [ "//third_party/abseil-cpp:absl" ]
@@ -682,20 +675,17 @@
       public_configs += invoker.public_configs
     }
 
-    # If absl_deps is [], no action is needed. If not [], then it needs to be
-    # converted to //third_party/abseil-cpp:absl when build_with_chromium=true
-    # otherwise it just needs to be added to deps.
-    if (absl_deps != []) {
-      if (!defined(deps)) {
-        deps = []
-      }
-      deps += absl_deps
-    }
+    assert(
+        !defined(absl_deps),
+        "`absl_deps` has been deprecated, add your Abseil dependencies to the `deps` variable.")
 
+    # Abseil dependencies need to be converted to //third_party/abseil-cpp:absl when build_with_chromium=true
     if (build_with_chromium && defined(deps)) {
-      absl_dependencies = filter_labels_include(deps, ["//third_party/abseil-cpp/*"])
+      absl_dependencies =
+          filter_labels_include(deps, [ "//third_party/abseil-cpp/*" ])
       if (absl_dependencies != []) {
-        filtered_deps = filter_labels_exclude(deps, ["//third_party/abseil-cpp/*"])
+        filtered_deps =
+            filter_labels_exclude(deps, [ "//third_party/abseil-cpp/*" ])
         deps = []
         deps = filtered_deps
         deps += [ "//third_party/abseil-cpp:absl" ]
@@ -777,20 +767,17 @@
       public_configs += invoker.public_configs
     }
 
-    # If absl_deps is [], no action is needed. If not [], then it needs to be
-    # converted to //third_party/abseil-cpp:absl when build_with_chromium=true
-    # otherwise it just needs to be added to deps.
-    if (absl_deps != []) {
-      if (!defined(deps)) {
-        deps = []
-      }
-      deps += absl_deps
-    }
+    assert(
+        !defined(absl_deps),
+        "`absl_deps` has been deprecated, add your Abseil dependencies to the `deps` variable.")
 
+    # Abseil dependencies need to be converted to //third_party/abseil-cpp:absl when build_with_chromium=true
     if (build_with_chromium && defined(deps)) {
-      absl_dependencies = filter_labels_include(deps, ["//third_party/abseil-cpp/*"])
+      absl_dependencies =
+          filter_labels_include(deps, [ "//third_party/abseil-cpp/*" ])
       if (absl_dependencies != []) {
-        filtered_deps = filter_labels_exclude(deps, ["//third_party/abseil-cpp/*"])
+        filtered_deps =
+            filter_labels_exclude(deps, [ "//third_party/abseil-cpp/*" ])
         deps = []
         deps = filtered_deps
         deps += [ "//third_party/abseil-cpp:absl" ]
@@ -922,20 +909,17 @@
       public_configs += invoker.public_configs
     }
 
-    # If absl_deps is [], no action is needed. If not [], then it needs to be
-    # converted to //third_party/abseil-cpp:absl when build_with_chromium=true
-    # otherwise it just needs to be added to deps.
-    if (absl_deps != []) {
-      if (!defined(deps)) {
-        deps = []
-      }
-      deps += absl_deps
-    }
+    assert(
+        !defined(absl_deps),
+        "`absl_deps` has been deprecated, add your Abseil dependencies to the `deps` variable.")
 
+    # Abseil dependencies need to be converted to //third_party/abseil-cpp:absl when build_with_chromium=true
     if (build_with_chromium && defined(deps)) {
-      absl_dependencies = filter_labels_include(deps, ["//third_party/abseil-cpp/*"])
+      absl_dependencies =
+          filter_labels_include(deps, [ "//third_party/abseil-cpp/*" ])
       if (absl_dependencies != []) {
-        filtered_deps = filter_labels_exclude(deps, ["//third_party/abseil-cpp/*"])
+        filtered_deps =
+            filter_labels_exclude(deps, [ "//third_party/abseil-cpp/*" ])
         deps = []
         deps = filtered_deps
         deps += [ "//third_party/abseil-cpp:absl" ]