Add plumbing for ServiceWorker's WorkerGlobalScope to talk back to client/chromium

Also adding a placeholder method (didHandleInstallEvent) to the Client class to notify that dispatching an InstallEvent is finished. (It'll be going to have more similar methods like didHandleFetchEvent etc)

The communication path from ServiceWorker's script context to chromium would look like:
WebCore::ServiceWorkerGlobalScope
=> WebCore::ServiceWorkerGlobalScopeClient
=> blink::WebServiceWorkerGlobalScopeClient
=> content::ServiceWorkerScriptContext
=> browser process

While the communication path from chromium to ServiceWorker's script context is:
browser process
=> content::ServiceWorkerScriptContext
=> blink::WebServiceWorkerGlobalScopeProxy
=> blink::ServiceWorkerGlobalScopeProxy
=> WebCore::ServiceWorkerGlobalScope

BUG=313530
TEST=to be added

Review URL: https://codereview.chromium.org/136113004

git-svn-id: svn://svn.chromium.org/blink/trunk@166384 bbb929c8-8fbe-4397-9dbb-9b2b20218538