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..0904d2db9acee06cda52129b5571c380dba3e074
--- /dev/null
+++ b/LayoutTests/http/tests/notifications/resources/click-forward-service-worker.js
@@ -0,0 +1,12 @@
+// 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) {
+    // FIXME: Include the notification's title in the confirmation reply.
+    messagePort.postMessage('The notification has been clicked on.');
+});