Implement WebPushProvider.unregister() in Chromium.
This is plumbing the call all the way to GCM in chrome/.
It is also updating the layout test push service to react properly if
unregister() is called multiple times or on a non-registered origin.
This is part of a multi-sided CL:
Part 1: https://codereview.chromium.org/
793913002/
Part 2: <this>
Part 3: https://codereview.chromium.org/
800503002/
BUG=440958
Review URL: https://codereview.chromium.org/
793403002
Cr-Commit-Position: refs/heads/master@{#308819}