Enable hermetic build for Google Chrome Mac official.

scripts/slave/recipe_modules/chromium_tests/chromium_chrome.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from . import steps
 
 RESULTS_URL = 'https://chromeperf.appspot.com'
 
 SPEC = {
   'builders': {
@@ skipping 30 matching lines @@
       ],
       'testing': {
         'platform': 'linux',
       },
       'tests': {
         steps.SizesStep(RESULTS_URL, 'Google Chrome Linux x64')
       },
     },
     'Google Chrome Mac': {
       'chromium_config': 'chromium_official',
-      'chromium_apply_config': ['mb'],
+      'chromium_apply_config': ['mb', 'force_mac_toolchain'],
       'gclient_config': 'chromium',
       'gclient_apply_config': ['chrome_internal'],
       'chromium_config_kwargs': {
         'BUILD_CONFIG': 'Release',
         'TARGET_BITS': 64,
       },
       'compile_targets': [
         'chrome',
       ],
       'testing': {
@@ skipping 17 matching lines @@
       ],
       'testing': {
         'platform': 'win',
       },
       'tests': {
         steps.SizesStep(RESULTS_URL, 'Google Chrome Win')
       },
     },
   },
 }