Tracking mock_paced_sender.h with a GN target

Untracked headers fly under the 'gn check' radar and in the long term
this can cause problems like unnoticed cyclic dependencies.

This cl creates a synthetic target for this header since no other
targets in webrtc/modules/pacing/BUILD.gn seem to be related to it.

BUG=webrtc:7649
NOTRY=True

Review-Url: https://codereview.webrtc.org/2887593003
Cr-Original-Commit-Position: refs/heads/master@{#19656}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 334f9e6a8d67f646b1a60a1683046224c351c5a4
diff --git a/call/BUILD.gn b/call/BUILD.gn
index 263f16d..19bc218 100644
--- a/call/BUILD.gn
+++ b/call/BUILD.gn
@@ -182,6 +182,7 @@
       "../modules/bitrate_controller",
       "../modules/congestion_controller:mock_congestion_controller",
       "../modules/pacing",
+      "../modules/pacing:mock_paced_sender",
       "../modules/rtp_rtcp",
       "../modules/rtp_rtcp:mock_rtp_rtcp",
       "../modules/utility:mock_process_thread",
diff --git a/modules/bitrate_controller/BUILD.gn b/modules/bitrate_controller/BUILD.gn
index 7c2063e..4ebf04c 100644
--- a/modules/bitrate_controller/BUILD.gn
+++ b/modules/bitrate_controller/BUILD.gn
@@ -61,6 +61,7 @@
       ":bitrate_controller",
       "../../test:field_trial",
       "../../test:test_support",
+      "../pacing:mock_paced_sender",
       "../remote_bitrate_estimator:remote_bitrate_estimator",
       "//testing/gmock",
     ]
diff --git a/modules/congestion_controller/BUILD.gn b/modules/congestion_controller/BUILD.gn
index 51abcd6..8c3f35b 100644
--- a/modules/congestion_controller/BUILD.gn
+++ b/modules/congestion_controller/BUILD.gn
@@ -97,6 +97,7 @@
       "../../test:field_trial",
       "../../test:test_support",
       "../bitrate_controller:bitrate_controller",
+      "../pacing:mock_paced_sender",
       "../pacing:pacing",
       "../remote_bitrate_estimator:remote_bitrate_estimator",
       "../rtp_rtcp:rtp_rtcp",
diff --git a/modules/pacing/BUILD.gn b/modules/pacing/BUILD.gn
index bc9d144..7884c01 100644
--- a/modules/pacing/BUILD.gn
+++ b/modules/pacing/BUILD.gn
@@ -75,4 +75,16 @@
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
   }
+
+  rtc_source_set("mock_paced_sender") {
+    testonly = true
+    sources = [
+      "mock/mock_paced_sender.h",
+    ]
+    deps = [
+      ":pacing",
+      "../../system_wrappers",
+      "../../test:test_support",
+    ]
+  }
 }