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..6198eb9a613c0b9ca80058321aeaf44181606f09 100644
--- a/content/public/common/BUILD.gn
+++ b/content/public/common/BUILD.gn
@@ -6,6 +6,7 @@ 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 +120,13 @@ mojom("mojo_bindings") {
     "service_worker_event_status.mojom",
   ]
 }
+
+source_set("feature_h264_with_openh264_ffmpeg") {
+  if (rtc_use_h264) {
+    defines = [ "BUILDFLAG_RTC_USE_H264" ]
+  }
+  sources = [
+    "feature_h264_with_openh264_ffmpeg.cc",
+    "feature_h264_with_openh264_ffmpeg.h",
+  ]
+}