WebRTC: Enable MB for Android bots.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[BUILDER_CACHE]/android_gn_rel",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 177 matching lines @@
       "GYP_CROSSCOMPILE": "1",
       "GYP_DEFINES": "OS=android component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[CACHE]/cipd/goma' target_arch=arm use_goma=1",
       "PATH": "[BUILDER_CACHE]/android_gn_rel/src/third_party/android_tools/sdk/platform-tools:[BUILDER_CACHE]/android_gn_rel/src/build/android:%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
-      "--root=[BUILDER_CACHE]/android_gn_rel/src",
+      "[BUILDER_CACHE]/android_gn_rel/src/tools/mb/mb.py",
       "gen",
-      "//out/Release",
-      "--args=is_debug=false target_os=\"android\" target_cpu=\"arm\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+      "-m",
+      "tryserver.webrtc",
+      "-b",
+      "android_gn_rel",
+      "--config-file",
+      "[BUILDER_CACHE]/android_gn_rel/src/webrtc/build/mb_config.pyl",
+      "--goma-dir",
+      "[CACHE]/cipd/goma",
+      "--gyp-script=[BUILDER_CACHE]/android_gn_rel/src/webrtc/build/gyp_webrtc.py",
+      "//out/Release"
     ],
-    "name": "gn"
+    "cwd": "[BUILDER_CACHE]/android_gn_rel/src",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
+    "name": "generate_build_files"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/compile.py",
       "--gsutil-py-path",
@@ skipping 30 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]