WebRTC: Enable OpenH264 for WebRTC standalone bots.

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_win_x64_gn_dbg.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE[build]\\bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "win_x64_gn_dbg",
@@ skipping 68 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]\\gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
       "GYP_CHROMIUM_NO_ACTION": "1",
-      "GYP_DEFINES": "chromium_win_pch=0 component=shared_library dcheck_always_on=1 fastbuild=1 gomadir='[BUILD]\\goma' target_arch=x64 use_goma=1"
+      "GYP_DEFINES": "chromium_win_pch=0 component=shared_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",
       "[DEPOT_TOOLS]\\gn.py",
       "--root=[SLAVE_BUILD]\\src",
       "gen",
       "//out/Debug_x64",
-      "--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 19 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]