Implement IsReadyToPay handling

content/public/android/java/src/org/chromium/IsReadyToPayService.aidl

+// 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.
+
+package org.chromium;
+
+/**
+ * Interface to determine whether a payment app
+ * is ready for payment.
+ */
+interface IsReadyToPayService {
+    /**
+     * Method that will be called on the Service to query
+     * whether the payment app is ready for payment.
+     *
+     * @param callback The callback to report back to the browser.
+     */
+    oneway void isReadyToPay(IsReadyToPayServiceCallback callback);
+}