Test that NotificationEvent.action is set correctly

LayoutTests/http/tests/notifications/resources/instrumentation-service-worker.js

 importScripts('/resources/testharness-helpers.js');
 
 // Copies the serializable attributes of |notification|.
 function cloneNotification(notification) {
     return JSON.parse(stringifyDOMObject(notification));
 }
 
 // Allows a document to exercise the Notifications API within a service worker by sending commands.
 var messagePort = null;
 
@@ skipping 59 matching lines @@
     // immediately by the Service Worker.
     if (event.notification.body.indexOf('ACTION:CLOSE') != -1)
         event.notification.close();
 
     // Notifications containing "ACTION:OPENWINDOW" in their message will attempt
     // to open a new window for an example URL.
     if (event.notification.body.indexOf('ACTION:OPENWINDOW') != -1)
         event.waitUntil(clients.openWindow('https://example.com/'));
 
     messagePort.postMessage({ command: 'click',
-                              notification: notificationCopy });
+                              notification: notificationCopy,
+                              action: event.action });
 });