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

1. [Blink] https://codereview.chromium.org/549093002/
2. [Chromium] https://codereview.chromium.org/554573002/
3. [Blink] this patch

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

Review URL: https://codereview.chromium.org/550093002

git-svn-id: svn://svn.chromium.org/blink/trunk@181783 bbb929c8-8fbe-4397-9dbb-9b2b20218538
ServiceWorker: Implement navigator.serviceWorker.getRegistration [1/3]

This is the first step towards implementing getRegistration method of
ServiceWorkerContainer. This patch adds definition of the method and
its callback type to WebServiceWorkerProvider.

[2/3] https://codereview.chromium.org/535753002/
[3/3] https://codereview.chromium.org/540823003/

BUG=404951
TEST=compile (tests will be added by subsequent patches)

Review URL: https://codereview.chromium.org/553983010

git-svn-id: svn://svn.chromium.org/blink/trunk@181784 bbb929c8-8fbe-4397-9dbb-9b2b20218538