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.
Getting the top level document's principal when constructing PaymentRequest,
then sending it to chrome process and saving it in nsIPaymentRequest.
Creating a new readonly attribute nsIPrincipal topLevelPrincipal in
nsIPaymentRequest for UI to query the top level document's principal
information.
