Add mac_optional_gpu_tests_rel bots to gpu related CLs.

tools/roll_webgl_conformance.py

 #!/usr/bin/env python
 # 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.
 
 import argparse
 import collections
 import logging
 import os
 import re
 import subprocess
 import sys
 import time
 
 extra_trybots = [
   {
     "mastername": "tryserver.chromium.win",
     "buildernames": ["win_optional_gpu_tests_rel"]
+  },
+  {
+    "mastername": "tryserver.chromium.mac",
+    "buildernames": ["mac_optional_gpu_tests_rel"]
   }
 ]
 
 SCRIPT_DIR = os.path.dirname(os.path.realpath(__file__))
 SRC_DIR = os.path.abspath(os.path.join(SCRIPT_DIR, os.pardir))
 sys.path.insert(0, os.path.join(SRC_DIR, 'build'))
 import find_depot_tools
 find_depot_tools.add_depot_tools_to_path()
 import roll_dep_svn
 from gclient import GClientKeywords
@@ skipping 342 matching lines @@
     logging.basicConfig(level=logging.ERROR)
 
   autoroller = AutoRoller(SRC_DIR)
   if args.abort:
     return autoroller.Abort()
   else:
     return autoroller.PrepareRoll(args.ignore_checks)
 
 if __name__ == '__main__':
   sys.exit(main())