Enable SwiftShader unit tests on Mac FYI try bots.

content/test/gpu/generate_buildbot_json.py

 #!/usr/bin/env python
 # Copyright 2016 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.
 
 """Script to generate chromium.gpu.json and chromium.gpu.fyi.json in
 the src/testing/buildbot directory. Maintaining these files by hand is
 too unwieldy.
 """
 
@@ skipping 1407 matching lines @@
     'args': [
       '--gtest_filter=*Detection*',
       '--use-gpu-in-tests'
     ]
   },
   'swiftshader_unittests': {
     'tester_configs': [
       {
         # Run this on the FYI waterfall and optional tryservers.
         'predicate': Predicates.FYI_AND_OPTIONAL,
-        'os_types': ['win', 'linux'],
+        'os_types': ['win', 'linux', 'mac'],
       },
     ],
     'disabled_tester_configs': [
       {
         'names': [
           'Linux ChromiumOS Ozone (Intel)',
         ],
       },
     ],
   },
@@ skipping 886 matching lines @@
   install_parent_links(WATERFALL)
   install_parent_links(V8_FYI_WATERFALL)
 
   generate_all_tests(FYI_WATERFALL, 'chromium.gpu.fyi.json')
   generate_all_tests(WATERFALL, 'chromium.gpu.json')
   generate_all_tests(V8_FYI_WATERFALL, 'client.v8.fyi.json')
   return 0
 
 if __name__ == "__main__":
   sys.exit(main())