ServiceWorker: Change the return value of ServiceWorkerRegistration::unregister to boolean (3/4)

1. [Blink] https://codereview.chromium.org/515083002/
2. [Chromium] https://codereview.chromium.org/516823003/
3. [Blink] This patch
4. [Chromium] https://codereview.chromium.org/509283003/

BUG=390894
TEST=N/A

Review URL: https://codereview.chromium.org/512163002

git-svn-id: svn://svn.chromium.org/blink/trunk@181295 bbb929c8-8fbe-4397-9dbb-9b2b20218538