ServiceWorker: Introduce ServiceWorkerMessageEvent to replace MessageEvent (2/3).

The message sent from Service Worker to its client will land at
navigator.serviceWorker.onmessage. The event type will be a new
ServiceWorkerMessageEvent which 'source' attribute could be
ServiceWorker or MessagePort.

This's the Chrome side CL to include ServiceWorker info when
dispatching message to renderer.

spec: https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#serviceworkermessage-event-section

BUG=498596

Review URL: https://codereview.chromium.org/1141813002

Cr-Commit-Position: refs/heads/master@{#336725}
6 files changed