Switch 'Win x64' builder on chromium waterfall to use MB.

scripts/slave/recipe_modules/chromium_tests/chromium.py

 # Copyright 2015 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
 
 SPEC = {
   'builders': {
     'Win': {
       'chromium_config': 'chromium',
@@ skipping 22 matching lines @@
       'gs_acl': 'public-read',
       'testing': {
         'platform': 'win',
       },
     },
     'Win x64': {
       'chromium_config': 'chromium',
       'chromium_apply_config': [
         'clobber',
         'isolation_mode_noop',
+        'mb',
         'ninja_confirm_noop',
         'no_dump_symbols',
       ],
       'gclient_config': 'chromium',
       'chromium_config_kwargs': {
         'BUILD_CONFIG': 'Release',
         'TARGET_BITS': 64,
       },
       'compile_targets': [
         'all',
@@ skipping 92 matching lines @@
       ],
       'archive_build': True,
       'gs_bucket': 'chromium-browser-snapshots',
       'gs_acl': 'public-read',
       'testing': {
         'platform': 'linux',
       },
     },
   },
 }