[ServiceWorker] Introduce isControlledByServiceWorker() in FrameLoaderClient and WebFrameClient

We need to check this in DocumentThreadableLoader not to send CORS preflight requests to the ServiceWorker.

Chromium side is implemented in https://codereview.chromium.org/553603002/.


BUG=411151

Review URL: https://codereview.chromium.org/546263002

git-svn-id: svn://svn.chromium.org/blink/trunk@181549 bbb929c8-8fbe-4397-9dbb-9b2b20218538