Removed references to mac_gpu, which has been removed

PRESUBMIT_test.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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 glob
 import json
 import os
 import re
 import subprocess
@@ skipping 602 matching lines @@
     for (filename, _, expected_error) in test_data:
       actual_error = PRESUBMIT._GetIDLParseError(input_api, filename)
       self.assertTrue(expected_error in str(actual_error),
                       "'%s' not found in '%s'" % (expected_error, actual_error))
 
 
 class TryServerMasterTest(unittest.TestCase):
   def testTryServerMasters(self):
     bots = {
         'tryserver.chromium.gpu': [
-            'mac_gpu',
-            'mac_gpu_triggered_tests',
             'linux_gpu',
             'linux_gpu_triggered_tests',
             'win_gpu',
             'win_gpu_triggered_tests',
         ],
         'tryserver.chromium.mac': [
             'ios_dbg_simulator',
             'ios_rel_device',
             'ios_rel_device_ninja',
             'mac_asan',
@@ skipping 79 matching lines @@
     }
     for master, bots in bots.iteritems():
       for bot in bots:
         self.assertEqual(master, PRESUBMIT.GetTryServerMasterForBot(bot),
                          'bot=%s: expected %s, computed %s' % (
             bot, master, PRESUBMIT.GetTryServerMasterForBot(bot)))
 
 
 if __name__ == '__main__':
   unittest.main()