ServiceWorker: Prevent premature garbage collection on ServiceWorkerRegistration

Before this patch, the ServiceWorkerRegistration object could be garbage
collected prematurely and it caused renderer crash.

This change keeps the registration object alive until stop() is called, and
notifies WebServiceWorkerRegistration of the stop event.


BUG=396400
TEST=https://codereview.chromium.org/466723002/

Review URL: https://codereview.chromium.org/467133002

git-svn-id: svn://svn.chromium.org/blink/trunk@180265 bbb929c8-8fbe-4397-9dbb-9b2b20218538