Actually hook up ServiceWorkerRegistration.showNotification()

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

Index: LayoutTests/http/tests/notifications/resources/click-forward-service-worker.js
diff --git a/LayoutTests/http/tests/notifications/resources/click-forward-service-worker.js b/LayoutTests/http/tests/notifications/resources/click-forward-service-worker.js
new file mode 100644
index 0000000000000000000000000000000000000000..0c9a7a778199ce8a9cc7b69f91a733dc3d77745f
--- /dev/null
+++ b/LayoutTests/http/tests/notifications/resources/click-forward-service-worker.js
@@ -0,0 +1,11 @@
+// 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);
+});