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This extends MessageEvent to allow ServiceWorker as a type of source
attribute according to the changes in HTML and replaces the existing
ServiceWorkerMessageEvent code usage with this extended MessageEvent.

Related spec issue: https://github.com/w3c/ServiceWorker/issues/989
Related spec change:
 - HTML: https://github.com/whatwg/html/pull/1944
 - SW: https://github.com/w3c/ServiceWorker/commit/6c68bd87bdfd153186a5b9904e31676c56730e2f
Related WPT: https://github.com/w3c/web-platform-tests/pull/4186

Intent to Deprecate and Remove: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/Xp9hmKyuOrI

BUG=659074

Review-Url: https://codereview.chromium.org/2466513002
Cr-Commit-Position: refs/heads/master@{#438545}