Handling PaymentResponse.prototype.onpayerdetailchange events

Each button will bring up the Payment Request UI window. When shown the payment sheet, use any details and hit pay.

When asked to retry the payment:

1. Change payer's name to "pass".

   PaymentRequestUpdateEvent is dispatched when payer name changes.

2. Change payer's email to "pass@pass.pass".

   PaymentRequestUpdateEvent is dispatched when payer email changes.

3. Change payer's phone to "+1-800-000-0000".

   PaymentRequestUpdateEvent is dispatched when payer phone changes.
4. DONE!