The blink half of the new ServiceWorker 'sync' event.

The browser side is https://codereview.chromium.org/205033002/

BUG=354112

Review URL: https://codereview.chromium.org/205013002

git-svn-id: svn://svn.chromium.org/blink/trunk@170165 bbb929c8-8fbe-4397-9dbb-9b2b20218538