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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