Adds tiny rtc::StringFormat utility

Bug: webrtc:11493
Change-Id: If11a0362dfa820e4464129d0ea58ff8bc4ce86bc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173323
Commit-Queue: Henrik Andreassson <henrika@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31043}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 60dda76..a61ede4 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -390,6 +390,8 @@
     "string_utils.h",
     "strings/string_builder.cc",
     "strings/string_builder.h",
+    "strings/string_format.cc",
+    "strings/string_format.h",
   ]
   deps = [
     ":checks",
@@ -819,12 +821,12 @@
     "net_helpers.cc",
     "net_helpers.h",
     "network.cc",
-    "network_route.cc",
     "network.h",
     "network_constants.cc",
     "network_constants.h",
     "network_monitor.cc",
     "network_monitor.h",
+    "network_route.cc",
     "network_route.h",
     "null_socket_server.cc",
     "null_socket_server.h",
@@ -1171,6 +1173,7 @@
       "string_utils_unittest.cc",
       "stringize_macros_unittest.cc",
       "strings/string_builder_unittest.cc",
+      "strings/string_format_unittest.cc",
       "swap_queue_unittest.cc",
       "thread_annotations_unittest.cc",
       "thread_checker_unittest.cc",
@@ -1307,8 +1310,8 @@
       "memory_usage_unittest.cc",
       "message_digest_unittest.cc",
       "nat_unittest.cc",
-      "network_unittest.cc",
       "network_route_unittest.cc",
+      "network_unittest.cc",
       "proxy_unittest.cc",
       "rolling_accumulator_unittest.cc",
       "rtc_certificate_generator_unittest.cc",