The Service Worker notificationclick event should carry a notification.

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

-// Sends a message to all clients when the notificationclick event gets received.
+// Sends a message when the notificationclick event gets received.
 var messagePort = null;
 
 addEventListener('message', function(event) {
     messagePort = event.data;
     messagePort.postMessage('ready');
 });
 
 addEventListener('notificationclick', function(event) {
-    // FIXME: Include the notification's title in the confirmation reply.
-    messagePort.postMessage('The notification has been clicked on.');
+    messagePort.postMessage('Clicked on Notification: ' + event.notification.title);
 });