PaymentRequest: Rename 'currencyCode' to 'currency' in CurrencyAmount.idl.

According to the references[1][2], 'currencyCode' property is renamed to
'currency' to allow flexible currency identifiers.

[1] https://w3c.github.io/browser-payment-api/specs/paymentrequest.html#h-currencyamount
[2] https://github.com/w3c/browser-payment-api/commit/563ecd594b5757f5175ee6ede3b41cdd8810d932

BUG=none

Review-Url: https://codereview.chromium.org/1981903002
Cr-Commit-Position: refs/heads/master@{#394822}
8 files changed