Add a License check to PRESUBMIT.py.

I've used the canned license check, and borrowed the regex from the chrome
presubmit. This catches a missing license, but doesn't cause the presubmit
to fail. If we want the presubmti to fail, I can try to figure that part out
as well.

** Presubmit Messages **
License must match:
.*? Copyright \(c\) 20\d\d The Chromium Authors\. All rights\.
.*? Use of this source code is governed by a BSD-style license that can be
.*? found in the LICENSE file\.

Found a bad license header in these files:
  PRESUBMIT.py \
  src/tracing_controller.js

BUG=trace-viewer:126

Review URL: https://codereview.appspot.com/7286045

git-svn-id: https://trace-viewer.googlecode.com/svn/trunk@206 0e6d7f2b-9903-5b78-7403-59d27f066143