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Support for events like click, close etc is now fully handled by the
ExtensionNotificationHandler like persistent web notifications.

The delegate is now just a little shim to hold the delegate id since it
is still required by the Notification class.

BUG=720345

Review-Url: https://codereview.chromium.org/2875673002
Cr-Commit-Position: refs/heads/master@{#473427}