ServiceWorker: Stop ServiceWorkerProviderContext from tracking statechange events
This is a cleanup CL. ServiceWorkerProviderContext does not have to track
statechange events:
* For ServiceWorkerGlobalScope, statechange events are queued up in
ServiceWorkerProviderHost until the worker thread is started and then
the events are directly delivered to WebServiceWorkerImpl.
* For (controlled) Document, the context does not care statechange
events on the associated registration.
BUG=n/a
Review URL: https://codereview.chromium.org/
1340243002
Cr-Commit-Position: refs/heads/master@{#348848}