PaymentApp: Implement invokePaymentApp() in renderer side.

content/test/data/payments/payment_app.js

+// 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.
+
+function sendResultToTest(data) {
+  clients.matchAll({includeUncontrolled: true}).then(clients => {
+    clients.forEach(client => {
+      if (client.url.indexOf('payment_app_invocation.html') != -1) {
+        client.postMessage(data);
+      }
+    });
+  });
+}
+
+function getMessageFromPaymentAppWindow() {
+  return new Promise((resolve, reject) => {
+    var listener = self.addEventListener('message', e => {
+      resolve(e.data);
+      self.removeEventListener(listener);
+    });
+  });
+}
+
+self.addEventListener('paymentrequest', e => {
+  var payment_app_window;
+
+  // SW ------------------ openWindow() -----------------> payment_app_window
+  // SW <---- postMessage('payment_app_window_ready') ---- payment_app_window
+  // SW ------- postMessage('payment_app_request') ------> payment_app_window
+  // SW <------ postMessage('payment_app_response') ------ payment_app_window
+  e.waitUntil(clients.openWindow('payment_app_window.html')
+                  .then(window_client => {
+                    payment_app_window = window_client;
+                    return getMessageFromPaymentAppWindow();
+                  })
+                  .then(message => {
+                    sendResultToTest(message);
+                    payment_app_window.postMessage('payment_app_request');
+                    return getMessageFromPaymentAppWindow();
+                  })
+                  .then(message => { sendResultToTest(message); }));
+});