Update masters after the tryserver split.

tools/valgrind/memcheck/PRESUBMIT.py

 # Copyright (c) 2011 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.
 
 """
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details on the presubmit API built into gcl.
 """
 
 import re
@@ skipping 61 matching lines @@
   return []
 
 def CheckChangeOnUpload(input_api, output_api):
   return CheckChange(input_api, output_api)
 
 def CheckChangeOnCommit(input_api, output_api):
   return CheckChange(input_api, output_api)
 
 def GetPreferredTryMasters(project, change):
   return {
-    'tryserver.chromium': {
+    'tryserver.chromium.linux': {
       'linux_valgrind': set(['defaulttests']),
+    },
+    'tryserver.chromium.mac': {
       'mac_valgrind': set(['defaulttests']),
     }
   }