ServiceWorker: Implement navigator.serviceWorker.getRegistration [3/3]

This completes the implementation of ServiceWorkerContainer#getRegistration().

Spec: https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#navigator-service-worker-getRegistration

[1/3] https://codereview.chromium.org/553983010/
[2/3] https://codereview.chromium.org/535753002/

BUG=404951
TEST=LayoutTests/http/tests/serviceworker/getregistration.html,webkit_unit_tests

Review URL: https://codereview.chromium.org/540823003

git-svn-id: svn://svn.chromium.org/blink/trunk@181952 bbb929c8-8fbe-4397-9dbb-9b2b20218538