ServiceWorker: Add 'updatefound' and version attributes on SWRegistration (Blink)

This change...
- adds 'onupdatefound' event and version attribtues (eg. '.installing') into
ServiceWorkerRegistration interface.
- adds WebServiceWorkerRegistrationProxy interface to talk to the registration
object from embedder.

[NOTE] ServiceWorkerRegistration is still disabled by the macro since Chromium
side is still actively being implemented:
https://codereview.chromium.org/427523002/

[Spec] https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-obj


BUG=396400
TEST=compile (layout tests will be added after enabling the registration interface)

Review URL: https://codereview.chromium.org/424693004

git-svn-id: svn://svn.chromium.org/blink/trunk@179903 bbb929c8-8fbe-4397-9dbb-9b2b20218538