Expose workerContextDestroyed() interface to ServiceWorker embedder

Current worker code has two notifications for worker termination:
* willDestroyWorkerContext - called right before WorkerGlobalScope is destroyed
* workerContextDestroyed - called after worker-related heap is cleaned up, currently most impl frees up worker thread etc here

The embedder (chromium) needed to be notified the latter too in order to 
properly shutdown the embedder-side worker context.

This depends on chromium-side patch:
https://codereview.chromium.org/264803007/

BUG=366617
TEST=content_browsertests:ServiceWorkerVersionBrowserTest.StartAndStop (with TSAN)
TEST=no crash by stopping SW in chrome://serviceworker-internals/

Review URL: https://codereview.chromium.org/262813004

git-svn-id: svn://svn.chromium.org/blink/trunk@173199 bbb929c8-8fbe-4397-9dbb-9b2b20218538