The Service Worker notificationclick event should carry a notification.

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

+// Sends a message to all clients when the notificationclick event gets received.

[Michael van Ouwerkerk, 2014/12/09 19:49:20]

Does it? Afaict it just messages to its single message port. Maybe there's only
one client :-)

[Peter Beverloo, 2014/12/10 14:11:09]

Done.

+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);
+});