Expose ServiceWorkerRegistration.showNotification() in Service Workers.

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);
-});