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: svn://svn.chromium.org/blink/trunk@185800 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed