Add swarming support for MB (and the GN bots) in the chromium recipes.

scripts/slave/recipe_modules/chromium_tests/chromium_win.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
 
 SPEC = {
   'settings': {
     'build_gs_bucket': 'chromium-win-archive',
   },
@@ skipping 180 matching lines @@
         'TARGET_BITS': 32,
       },
       'bot_type': 'builder_tester',
       'test_generators': [
         steps.generate_gtest,
         steps.generate_script,
       ],
       'testing': {
         'platform': 'win',
       },
+      'enable_swarming': True,
     },
   },
 }