Expose the notificationclick and notificationerror events on ServiceWorkerGlobalScope

This patch introduces the notificationclick and notificationerror
events on the ServiceWorkerGlobalScope, and provides the necessary
plumbing to be able to invoke them from the embedder.

BUG=432527

Review URL: https://codereview.chromium.org/720873002

git-svn-id: svn://svn.chromium.org/blink/trunk@185313 bbb929c8-8fbe-4397-9dbb-9b2b20218538