Service Worker: respondWith() invokes stopImmediatePropagation()

As per spec discussion:
https://github.com/slightlyoff/ServiceWorker/issues/51#issuecomment-51172079

BUG=432827

Review URL: https://codereview.chromium.org/731433002

git-svn-id: svn://svn.chromium.org/blink/trunk@185357 bbb929c8-8fbe-4397-9dbb-9b2b20218538