WebRTC: Add gyp_script parameter to ios recipe module's run_mb call.

scripts/slave/recipes/webrtc/ios.expected/gn_build.json

 [
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "name": "host and tools info",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br />OS X 1.2.3, Xcode 6.7.8 (5D342509a)@@@",
@@ skipping 219 matching lines @@
       "[BUILDER_CACHE]/ios/src/tools/mb/mb.py",
       "gen",
       "-m",
       "chromium.fake",
       "-b",
       "ios",
       "--config-file",
       "[BUILDER_CACHE]/ios/src/webrtc/build/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
+      "--gyp-script=[BUILDER_CACHE]/ios/src/webrtc/build/gyp_webrtc.py",
       "//out/Debug-iphoneos"
     ],
     "cwd": "[BUILDER_CACHE]/ios/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files"
   },
   {
     "cmd": [
       "ninja",
       "-C",
       "[BUILDER_CACHE]/ios/src/out/Debug-iphoneos"
     ],
     "cwd": "[BUILDER_CACHE]/ios/src/out/Debug-iphoneos",
     "name": "compile"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]