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The respondWith() method is used by the payment app to provide a
PaymentAppResponse when the payment successfully completes.

Related Spec Link:
  https://w3c.github.io/webpayments-payment-apps-api/#idl-def-paymentrequestevent

See the other CLs in this series:
  [1/3] https://codereview.chromium.org/2715663002/ (RespondWithObserver)
  [2/3] This patch
  [3/3] https://codereview.chromium.org/2718013004/ (content side and test)

BUG=661608
TEST=payment_app_browsertest.cc

Review-Url: https://codereview.chromium.org/2705293010
Cr-Commit-Position: refs/heads/master@{#457841}