Actually hook up ServiceWorkerRegistration.showNotification()

LayoutTests/http/tests/notifications/resources/click-forward-service-worker.js

+// Sends a message to all clients when the notificationclick event gets received.
+var messagePort = null;
+
+addEventListener('message', function(event) {
+    messagePort = event.data;
+    messagePort.postMessage('ready');
+});
+
+addEventListener('notificationclick', function(event) {
+    messagePort.postMessage('Clicked on Notification: ' + event.notification.title);
+});