WebRTC: Enable OpenH264 for WebRTC standalone bots.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE[build]/bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "mac_swarming",
@@ skipping 79 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
-      "GYP_DEFINES": "clang=1 component=shared_library dcheck_always_on=1 fastbuild=1 gomadir='[BUILD]/goma' target_arch=x64 test_isolation_mode=prepare use_goma=1"
+      "GYP_DEFINES": "clang=1 component=shared_library dcheck_always_on=1 fastbuild=1 ffmpeg_branding=Chrome gomadir='[BUILD]/goma' rtc_use_264=1 target_arch=x64 test_isolation_mode=prepare use_goma=1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/tools/swarming_client/swarming.py",
       "--version"
     ],
@@ skipping 2065 matching lines @@
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LINK@shard #0 isolated out@blah@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]