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

testing_support/filesystem_mock.py

Index: testing_support/filesystem_mock.py
diff --git a/testing_support/filesystem_mock.py b/testing_support/filesystem_mock.py
index b45ea8e37409a766009e1afbb4e10b0dad357a3b..7a44dabb3287b3b44708bfb68aa3aa3d03ff6232 100644
--- a/testing_support/filesystem_mock.py
+++ b/testing_support/filesystem_mock.py
@@ -86,9 +86,9 @@ class MockFileSystem(object):
       _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):]