Update masters after the tryserver split.

extensions/browser/PRESUBMIT.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.
 
 """Chromium presubmit script for src/extensions/browser.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details on the presubmit API built into gcl.
 """
 
 import sys
 
 def GetPreferredTryMasters(project, change):
   return {
-    'tryserver.chromium': {
+    'tryserver.chromium.linux': {
       'linux_chromium_chromeos_rel': set(['defaulttests']),
     }
   }
 
 def _CreateHistogramValueChecker(input_api, output_api):
   original_sys_path = sys.path
 
   try:
     sys.path.append(input_api.os_path.join(
         input_api.PresubmitLocalPath(), '..', '..', 'tools',
         'strict_enum_value_checker'))
     from strict_enum_value_checker import StrictEnumValueChecker
   finally:
     sys.path = original_sys_path
 
   return StrictEnumValueChecker(input_api, output_api,
       start_marker='enum HistogramValue {', end_marker='  // Last entry:',
       path='extensions/browser/extension_function_histogram_value.h')
 
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results += _CreateHistogramValueChecker(input_api, output_api).Run()
   results += input_api.canned_checks.CheckPatchFormatted(input_api, output_api)
   return results
 
 def CheckChangeOnCommit(input_api, output_api):
   return _CreateHistogramValueChecker(input_api, output_api).Run()