Enable H.264 video WebRTC behind run-time flag and add WebRtcBrowserTest for H.264

content/public/common/BUILD.gn

Index: content/public/common/BUILD.gn
diff --git a/content/public/common/BUILD.gn b/content/public/common/BUILD.gn
index b8a7cd1f699f2847747ef23ae29c509a19fe9a11..83d47d6e07a9e51bba252a4e1bd29c34dc9105e9 100644
--- a/content/public/common/BUILD.gn
+++ b/content/public/common/BUILD.gn
@@ -2,10 +2,12 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/buildflag_header.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/features.gni")
 import("//content/common/common.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
+import("//third_party/webrtc/build/webrtc.gni")
 
 # See //content/BUILD.gn for how this works.
 group("common") {
@@ -119,3 +121,19 @@ mojom("mojo_bindings") {
     "service_worker_event_status.mojom",
   ]
 }
+
+buildflag_header("features") {
+  header = "features.h"
+
+  flags = [ "RTC_USE_H264=$rtc_use_h264" ]
+}
+
+source_set("feature_h264_with_openh264_ffmpeg") {
+  deps = [
+    ":features",
+  ]
+  sources = [
+    "feature_h264_with_openh264_ffmpeg.cc",
+    "feature_h264_with_openh264_ffmpeg.h",
+  ]
+}