iframes with allowpaymentrequest attribute are allowed to make payment requests.

Specification: https://w3c.github.io/browser-payment-api/

There are some circumstances where a cross-origin iframe
wants to make a payment request. A cross-origin iframe
needs explicit permission from the embedding page to invoke
the payment request API.

BUG=652148

Review-Url: https://chromiumcodereview.appspot.com/2394473002
Cr-Commit-Position: refs/heads/master@{#426513}
17 files changed