Revert of Make ServiceWorker an EventTarget and dispatch statechange to it. (https://codereview.chromium.org/236823002/)

Reason for revert:
[Gardening] This patch seemed to break http/tests/serviceworker/state.html

Dashboard:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=http/tests/serviceworker/state.html

Original issue's description:
> Make ServiceWorker an EventTarget and dispatch statechange to it.
> 
> Some infelicities with the event when reusing a service worker, and
> unregistering service workers, are noted in FIXMEs in the test. Those
> fixes must come later.
> 
> BUG=363413
> R=kinuko@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=171525

TBR=kinuko@chromium.org,dominicc@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=363413

Review URL: https://codereview.chromium.org/238653002

git-svn-id: svn://svn.chromium.org/blink/trunk@171532 bbb929c8-8fbe-4397-9dbb-9b2b20218538