[paymentrequest] Need a "really closed" event (#45)

Currently steps 3 and 4 of the [`complete()` method](http://wicg.github.io/paymentrequest/specs/paymentrequest.html#complete) mean that the user agent's UI can get torn down asynchronously. This is designed to allow the UA to use some kind of animation to indicate the success or failure of the payment request before dismissing the UI.

If the web site navigates the page before the UI disappears this may appear incongruous. We should decide whether the UA should fire an event to signal that the UI is gone and everything is complete to address this. 

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/paymentrequest/issues/45