The Service Worker notificationclick event should carry a notification.

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

+// Sends a message when the notificationclick event gets received, and closes the
+// notification which was shown from that event.
+var messagePort = null;
+
+addEventListener('message', function(event) {
+    messagePort = event.data;
+    messagePort.postMessage('ready');
+});
+
+addEventListener('notificationclick', function(event) {
+    event.notification.close();
+    messagePort.postMessage('Clicked on Notification: ' + event.notification.title);
+});