Introduce the NotificationEvent object.

Notification events delivered to Service Workers (notificationclick
and notificationerror) will receive an instance of the Notification
object which they were triggered for, represented through a
NotificationEvent. It's of the SW ExtendableEvent type.

BUG=432527

Review URL: https://codereview.chromium.org/718093002

git-svn-id: svn://svn.chromium.org/blink/trunk@185296 bbb929c8-8fbe-4397-9dbb-9b2b20218538