Ensure JavaScript equality when serviceWorker.register resolves to an existing worker [1/3]

If there's an existing registration for the scope and script url,
serviceWorker.register resolves to the active worker for that registration. This
patch modifies the dispatcher to reuse an existing ServiceWorkerHandle, so
service workers that are the same have JavaScript equality.

Three-sided patch:
[1] Blink: ServiceWorker::from can be called from CallbackPromiseAdapter
for a SW that was already registered and reached the proxy "ready" state
[2] Chromium: https://codereview.chromium.org/412833002/
[3] Blink: https://codereview.chromium.org/411863003

BUG=395928

Review URL: https://codereview.chromium.org/409403002

git-svn-id: svn://svn.chromium.org/blink/trunk@178810 bbb929c8-8fbe-4397-9dbb-9b2b20218538