WebRTC: Enable OpenH264 for WebRTC standalone bots.

scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_linux64_debug__gn_.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE[build]/bot_update.py",
       "--master",
       "client.webrtc",
       "--builder",
       "Linux64 Debug (GN)",
@@ skipping 62 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
       "GYP_CHROMIUM_NO_ACTION": "1",
-      "GYP_DEFINES": "component=shared_library dcheck_always_on=1 gomadir='[BUILD]/goma' target_arch=x64 use_goma=1"
+      "GYP_DEFINES": "component=shared_library dcheck_always_on=1 ffmpeg_branding=Chrome gomadir='[BUILD]/goma' rtc_use_264=1 target_arch=x64 use_goma=1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gn.py",
       "--root=[SLAVE_BUILD]/src",
       "gen",
       "//out/Debug",
-      "--args=is_debug=true target_cpu=\"x64\" use_goma=true goma_dir=\"[BUILD]/goma\""
+      "--args=ffmpeg_branding=\"Chrome\" rtc_use_h264=true is_debug=true target_cpu=\"x64\" use_goma=true goma_dir=\"[BUILD]/goma\""
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "gn"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/compile.py",
       "--target",
@@ skipping 17 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]