Fix relative file: paths in OWNERS with roots other than '/'

testing_support/filesystem_mock.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.
 
 import errno
 import fnmatch
 import os
 import re
 import StringIO
 
@@ skipping 68 matching lines @@
 
   def open_for_reading(self, path):
     return StringIO.StringIO(self.read_binary_file(path))
 
   def read_binary_file(self, path):
     # Intentionally raises KeyError if we don't recognize the path.
     if self.files[path] is None:
       _RaiseNotFound(path)
     return self.files[path]
 
-  @staticmethod
-  def relpath(path, base):
+  def relpath(self, path, base):
     # This implementation is wrong in many ways; assert to check them for now.
+    if not base.endswith(self.sep):
+      base += self.sep
     assert path.startswith(base)
-    assert base.endswith('/')
     return path[len(base):]