Make NotificationEvent constructable.

LayoutTests/http/tests/notifications/resources/serviceworker-notification-event.js

 importScripts('../../serviceworker/resources/worker-testharness.js');
 importScripts('/resources/testharness-helpers.js');
 
 test(function() {
     assert_true('NotificationEvent' in self);
 
-}, 'NotificationEvent is exposed.');
+    var event = new NotificationEvent('NotificationEvent');
+    assert_equals(event.type, 'NotificationEvent');
+    assert_own_property(event, 'notification');
+    assert_inherits(event, 'waitUntil');
+
+}, 'NotificationEvent is exposed, and has the expected interface.');
 
 test(function() {
     assert_own_property(self, 'onnotificationclick', 'The notificationclick event exists.');
     assert_own_property(self, 'onnotificationerror', 'The notificationerror event exists.');
 
 }, 'The notificationclick and notificationerror events exist on the global scope.');