Add shippingOption to PaymentResponse

PaymentRequest lets web developers access the shipping option String
through the PaymentRequest object. As it is a part of information
necessary to complete a transaction, it makes sense to place this
information in PaymentResponse as well.

https://w3c.github.io/browser-payment-api/#paymentresponse-interface
https://github.com/w3c/browser-payment-api/commit/47db154ac80a1b1b6bfde4627eab6f496e47670c

* All tests for PaymentRequest::OnPaymentResponse() have been moved into
  OnPaymentResponseTest.cpp.
* The shipping_option_id field in Mojo is renamed to shipping_option to
  match the JavaScript API.

BUG=587995

Review-Url: https://codereview.chromium.org/2073603002
Cr-Commit-Position: refs/heads/master@{#400545}
13 files changed