Actually start ServiceWorkerThread in EmbeddedWorker

- Implement actual code to start ServiceWorkerThread and
  ServiceWorkerGlobalScope in WebEmbeddedWorkerImpl::onScriptLoaderFinished().
- Also adds ServiceWorkerGlobalScopeProxy class, which implements
  WebServiceWorkerContextProxy and WorkerLoaderProxy.
  This does mostly just relaying messages to/from WorkerGlobalScope
  and ServiceWorkerContextClient implementation in Chromium.

BUG=313530

Review URL: https://codereview.chromium.org/91173002

git-svn-id: svn://svn.chromium.org/blink/trunk@163495 bbb929c8-8fbe-4397-9dbb-9b2b20218538