Mark the Notification as being closed before dispatching the event.
Mark the Notification as being closed before dispatching the event.

This avoids a situation in which we can crash when the developer calls
the Notification.close() method in the "onclose" event of a Notification.

BUG=435201

Review URL: https://codereview.chromium.org/745713003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@185800 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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