PaymentApp: Add PaymentAppResponse interface.

third_party/WebKit/Source/modules/payments/PaymentAppResponse.idl

Index: third_party/WebKit/Source/modules/payments/PaymentAppResponse.idl
diff --git a/third_party/WebKit/Source/modules/payments/PaymentAppResponse.idl b/third_party/WebKit/Source/modules/payments/PaymentAppResponse.idl
new file mode 100644
index 0000000000000000000000000000000000000000..b68b76d3cb5ced55420fe1aeaa2a2828042f8937
--- /dev/null
+++ b/third_party/WebKit/Source/modules/payments/PaymentAppResponse.idl
@@ -0,0 +1,10 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// https://w3c.github.io/webpayments-payment-apps-api/#idl-def-paymentappresponse
+
+dictionary PaymentAppResponse {
+    DOMString methodName;
+};
+~