Introduce layout tests for SWR.getNotifications()

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;
 
 addEventListener('message', function(workerEvent) {
     messagePort = workerEvent.data;
 
     // Listen to incoming commands on the message port.
     messagePort.onmessage = function(event) {
         if (typeof event.data != 'object' || !event.data.command)
             return;
 
         switch (event.data.command) {
             case 'permission':
                 messagePort.postMessage({ command: event.data.command,
                                           value: Notification.permission });
                 break;
 
             case 'show':
                 registration.showNotification(event.data.title, event.data.options).then(function() {
                     messagePort.postMessage({ command: event.data.command,
                                               success: true });
                 }, function(error) {
                     messagePort.postMessage({ command: event.data.command,
                                               success: false,
                                               message: error.message });
                 });
                 break;
 
+            case 'get':
+                var filter = {};
+                if (typeof (event.data.filter) !== 'undefined')
+                    filter = event.data.filter;
+
+                registration.getNotifications(filter).then(function(notifications) {
+                    var clonedNotifications = [];
+                    for (var notification of notifications)
+                        clonedNotifications.push(cloneNotification(notification));
+
+                    messagePort.postMessage({ command: event.data.command,
+                                              success: true,
+                                              notifications: clonedNotifications });
+                }, function(error) {
+                    messagePort.postMessage({ command: event.data.command,
+                                              success: false,
+                                              message: error.message });
+                });
+                break;
+
             default:
                 messagePort.postMessage({ command: 'error', message: 'Invalid command: ' + event.data.command });
                 break;
         }
     };
 
     // Notify the controller that the worker is now available.
     messagePort.postMessage('ready');
 });
 
 addEventListener('notificationclick', function(event) {
-    // Copies the serializable attributes of the Notification instance on |event|.
-    var notificationCopy = JSON.parse(stringifyDOMObject(event.notification));
+    var notificationCopy = cloneNotification(event.notification);
 
     // Notifications containing "ACTION:CLOSE" in their message will be closed
     // 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 });
 });