ServiceWorkerRegistration::unregister returns false when the registration is already unregistered. (1/3)

1. [Blink] this patch
2. [Chromium] https://codereview.chromium.org/554573002/
3. [Blink] https://codereview.chromium.org/550093002/

BUG=390894
TEST=run_webkit_tests.py http/tests/serviceworker/unregister.html

Review URL: https://codereview.chromium.org/549093002

git-svn-id: svn://svn.chromium.org/blink/trunk@181692 bbb929c8-8fbe-4397-9dbb-9b2b20218538