Only expose Notification.requestPermission() on the Window global.

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 35 matching lines @@
                     messagePort.postMessage({ command: event.data.command,
                                               success: true,
                                               notifications: clonedNotifications });
                 }, function(error) {
                     messagePort.postMessage({ command: event.data.command,
                                               success: false,
                                               message: error.message });
                 });
                 break;
 
+            case 'request-permission-exists':
+                messagePort.postMessage({ command: event.data.command,
+                                          value: 'requestPermission' in Notification });
+                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) {
     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,
                               action: event.action });
 });