[Payment] Clean up PaymentResponse::toJSONForBinding

When https://codereview.chromium.org/2476393003/ is landed.
We could clean up toJSONForBinding.
BUG=648051

Review-Url: https://codereview.chromium.org/2491513002
Cr-Commit-Position: refs/heads/master@{#431269}
1 file changed