Add a presubmit check that warns about declaring Singleton<T> in header files

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 11 matching lines @@
     self.os_path = os.path
     self.python_executable = sys.executable
     self.subprocess = subprocess
     self.files = []
     self.is_committing = False
     self.change = MockChange([])
 
   def AffectedFiles(self, file_filter=None):
     return self.files
 
+  def AffectedSourceFiles(self, file_filter=None):
+    return self.files
+
   def PresubmitLocalPath(self):
     return os.path.dirname(__file__)
 
   def ReadFile(self, filename, mode='rU'):
     for file_ in self.files:
       if file_.LocalPath() == filename:
         return '\n'.join(file_.NewContents())
     # Otherwise, file is not in our mock API.
     raise IOError, "No such file or directory: '%s'" % filename
 
@@ skipping 59 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