[Findit] Add a Rietveld client to post a message to codereview of a culprit.

appengine/findit/common/codereview.py

+# Copyright 2016 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.
+
+
+# pylint: disable=E0711, W0613, R0201
+class CodeReview(object):  # pragma: no cover
+  """An interface to interact with code review."""
+
+  def PostMessage(self, codereview_url, message):
+    """ Posts the given message to the CL codereview of the specified url.
+
+    args:
+      codereview_url(str): The url to a CL codereview.
+      message(str): The message to be posted to the codereview.
+    """
+    raise NotImplemented()