[paymentrequest] API needs equivalent of the paymentAuthorizationViewController:didAuthorizePayment:completion: callback (#23)

Hi,

In PassKit (the Apple Pay API) the payment sheet stays up until the app calls this completion handler with either `PKPaymentAuthorizationStatus.Success` or `PKPaymentAuthorizationStatus.Failure`. (You can read more about this in the [official docs](https://developer.apple.com/library/ios/documentation/PassKit/Reference/PKPaymentAuthorizationViewControllerDelegate_Ref/index.html#//apple_ref/doc/c_ref/PKPaymentAuthorizationStatus), although you might find [this NSHipster blog post](http://nshipster.com/apple-pay/#pkpaymentauthorizationviewcontrollerdelegate) to be more straightforward.)

In order for this JS API to work with PassKit, the UA needs to expose a method that the merchant calls to signify success or failure after it does whatever it does with the instrumentResponse.

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/paymentrequest/issues/23