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

content/public/renderer/BUILD.gn

Index: content/public/renderer/BUILD.gn
diff --git a/content/public/renderer/BUILD.gn b/content/public/renderer/BUILD.gn
index 719eadf6771980e13dd3748f7c48c017414328ae..a169d0376754197685354f0d6826f9b317721e67 100644
--- a/content/public/renderer/BUILD.gn
+++ b/content/public/renderer/BUILD.gn
@@ -4,6 +4,7 @@
 
 import("//build/config/features.gni")
 import("//content/renderer/renderer.gni")
+import("//third_party/webrtc/build/webrtc.gni")
 
 # See //content/BUILD.gn for how this works.
 group("renderer") {
@@ -37,6 +38,7 @@ source_set("renderer_sources") {
     "//ui/gfx",
     "//v8",
   ]
+  defines = []
 
   allow_circular_includes_from = [
     # This target is a pair with content/renderer. They always go together and
@@ -49,7 +51,13 @@ source_set("renderer_sources") {
         rebase_path(content_renderer_gypi_values.public_renderer_webrtc_sources,
                     ".",
                     "//content")
-    deps += [ "//third_party/webrtc" ]
+    deps += [
+      "//content/public/common:feature_h264_with_openh264_ffmpeg",
+      "//third_party/webrtc",
+    ]
+    if (rtc_use_h264) {
+      defines += [ "BUILDFLAG_RTC_USE_H264" ]
+    }
   }
 
   if (enable_plugins) {