[Extern Generation] Add a presubmit script to check externs not being updated

PRESUBMIT_test_mocks.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.
 
 import json
 import os
 import re
 import subprocess
 import sys
 
@@ skipping 91 matching lines @@
 
   def ChangedContents(self):
     return self._changed_contents
 
   def NewContents(self):
     return self._new_contents
 
   def LocalPath(self):
     return self._local_path
 
+  def AbsoluteLocalPath(self):
+    return self._local_path
+
   def rfind(self, p):
     """os.path.basename is called on MockFile so we need an rfind method."""
     return self._local_path.rfind(p)
 
   def __getitem__(self, i):
     """os.path.basename is called on MockFile so we need a get method."""
     return self._local_path[i]
 
 
 class MockAffectedFile(MockFile):
   def AbsoluteLocalPath(self):
     return self._local_path
 
 
 class MockChange(object):
   """Mock class for Change class.
 
   This class can be used in presubmit unittests to mock the query of the
   current change.
   """
 
   def __init__(self, changed_files):
     self._changed_files = changed_files
 
   def LocalPaths(self):
     return self._changed_files