Fix EmbeddedWorker code so that ServiceWorker test code works

* Changed to call WorkerReportingProxy::workerGlobalScopeStarted() after
  didStartWorkerRunLoop() is notified so that chromium side can get the
  correct worker thread's ID within workerGlobalScopeStarted()
* Added missing WebFrameClient::didFinishDocumentLoad() override in
  WebEmbeddedWorkerImpl so that it can correctly wait for document
  loading before loading the worker script
* Separated out the WebEmbeddedWorkerImpl's Loader::load() method
  from its ctor so that m_mainScriptLoader is always non-null when
  notifyFinished() is called.
* Tentatively changed to use ResourceRequest::TargetIsSharedWorker instead of
  ResourceRequest::TargetIsServiceWorker until more chrome side plumbing is done

BUG=313530
TEST=to be added

Review URL: https://codereview.chromium.org/126153003

git-svn-id: svn://svn.chromium.org/blink/trunk@164611 bbb929c8-8fbe-4397-9dbb-9b2b20218538