1. Add "optional Promise<PaymentDetailsUpdate> detailsPromise" as a parameter
of PaymentRequest.show() in PaymentRequest.webidl.
2. Let PaymentRequest inherit from PromiseNativeHandler, and implement the
ResolvedCallback() and RejectedCallback() to handle the PaymentDetailsUpdate
promise.
3. Update PaymentRequest.show() implementation. If PaymentDetailsUpdate Promise
is not nullptr, the show request would not be transferred to chrome process
immediately until the promise is resolved/rejected.
4. Update selectedShippingOption when requestShipping is true.
5. Change the PaymentMethod id validation sequence according to the spec.
This patch implements currency validation algorithm according to the spec
https://w3c.github.io/payment-request/#validity-checkers.
1. amount.currencySystem must be "urn:iso:std:iso:4217".
2. amount.currency is valid with following criteria
1. The currency length must be 3.
2. The currency contains any character that must be in the range "A" to
"Z"(U+0041 to U+005A) or the range "a" to "z"(U+0061 to U+007A).
According to the spec, converting the currency to upper case and save it in
nsIPaymentRequest.
