[Closure Externs] Fix presubmit check

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 122 matching lines @@
 
   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
+
+  def AffectedFiles(self, include_dirs=False, include_deletes=True,
+                    file_filter=None):
+    return self._changed_files