WebRTC: Enable OpenH264 for WebRTC standalone bots.

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_win_x64_rel.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE[build]\\bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "win_x64_rel",
@@ skipping 67 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]\\gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
-      "GYP_DEFINES": "chromium_win_pch=0 component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[BUILD]\\goma' target_arch=x64 use_goma=1"
+      "GYP_DEFINES": "chromium_win_pch=0 component=static_library dcheck_always_on=1 fastbuild=1 ffmpeg_branding=Chrome gomadir='[BUILD]\\goma' rtc_use_264=1 target_arch=x64 use_goma=1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\compile.py",
       "--target",
       "Release_x64",
@@ skipping 377 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "webrtc_nonparallel_tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]