WebRTC: Use custom mb_config_path for standalone bots.

scripts/slave/recipes/webrtc/ios.py

Index: scripts/slave/recipes/webrtc/ios.py
diff --git a/scripts/slave/recipes/webrtc/ios.py b/scripts/slave/recipes/webrtc/ios.py
index 6f4d6a90d9772497bd796ca5faa79ef0adf8d171..98b93e8ee0da971957514472ee85f0ec2e702471 100644
--- a/scripts/slave/recipes/webrtc/ios.py
+++ b/scripts/slave/recipes/webrtc/ios.py
@@ -36,7 +36,12 @@ def RunSteps(api):
   api.ios.read_build_config(build_config_dir=build_config_dir,
                             include_dir=include_dir,
                             buildername=buildername)
-  api.ios.build()
+  mb_config_path = api.path['checkout'].join(
+      'webrtc',
+      'build',
+      'mb_config.pyl',
+  )
+  api.ios.build(mb_config_path=mb_config_path)
   api.ios.test()
 
 
@@ -76,6 +81,31 @@ def GenTests(api):
   )
 
   yield (
+    api.test('gn_build')
+    + api.platform('mac', 64)
+    + api.properties(
+      buildername='ios',
+      buildnumber='0',
+      mastername='chromium.fake',
+      slavename='fake-vm',
+    )
+    + api.ios.make_test_build_config({
+      'xcode version': 'fake xcode version',
+      'GYP_DEFINES': {
+      },
+      "gn_args": [
+        "is_debug=true"
+      ],
+      "mb_type": "gn",
+      'compiler': 'ninja',
+      'configuration': 'Debug',
+      'sdk': 'iphoneos8.0',
+      'tests': [
+      ],
+    })
+  )
+
+  yield (
     api.test('no_tests')
     + api.platform('mac', 64)
     + api.properties(