PaymentApp: Implement invokePaymentApp() in renderer side.

third_party/WebKit/LayoutTests/http/tests/payments/chromium/resources/payment-app.js

+var testerResolver;
+var port;
+
+self.addEventListener('message', e => {
+  if (e.data.port) {
+    port = e.data.port;
+    port.postMessage('payment_app_ready');
+  } else {
+    // If the message event is initiated from tester, ignores this event.

[falken, 2017/01/24 02:00:51]

What is "tester", and why is it sending a message which is ignored?

+    return;
+  }
+  e.waitUntil(new Promise((resolve) => {
+        testerResolver = resolve;
+      })
+    .then(() => {
+        port.postMessage('payment_app_quit');
+      }));
+});
+
+function getMessageFromPaymentAppWindow() {
+  return new Promise((resolve, reject) => {
+      var listener = self.addEventListener('message', e => {
+          resolve(e.data);
+          self.removeEventListener(listener);

[falken, 2017/01/24 02:00:51]

nit: Having multiple message handlers seems more complicated than needed. Would
it be clearer if there is just one message handler that does the intended thing
based on the data in the message?

+        });
+    });
+}
+
+self.addEventListener('paymentrequest', e => {
+  var paymentAppWindow;
+
+  e.waitUntil(clients.openWindow('payment-app-window.html')
+    .then(windowClient => {
+        paymentAppWindow = windowClient;
+        return getMessageFromPaymentAppWindow();
+      })
+    .then(message => {
+        if (message == 'payment_app_window_ready')

[falken, 2017/01/24 02:00:51]

What other message could arrive here? Could this be an assert instead?

+          paymentAppWindow.postMessage('payment_app_request');
+        return getMessageFromPaymentAppWindow();
+      })
+    .then(message => {
+        if (message == 'payment_app_response') {

[falken, 2017/01/24 02:00:51]

Same.

+          // Disconnect tester.
+          testerResolver();
+        }
+      }));
+});