Add support for communicating billing address to merchant (e.g., for VAT computation) #314

This issue is the payment handler complement to Payment Request API issue 27:
w3c/payment-request#27

Example flow:

• The user selects a payment handler.
• Within the payment handler, the user selects a payment instrument with associated billing address.
• The payment handler informs the browser that the user has selected a billing address. The payment handler waits for a response (read: promise to resolve) that may update the total. The payment handler will not complete the transaction until hearing back from the merchant.
• The browser fires the onpaymentmethodchange event to let the merchant know the billing address has changed.
• The merchant responds with a (possibly null) update of the total.
• The browser gives the (possibly updated) total back to the payment handler.
• The payment handler displays the updated total and allows the user to proceed.

Let's keep an eye on this pull request:
w3c/payment-request#749

And this one:
w3c/payment-method-basic-card#53