bindings: Fixes layouttests when moving attributes to prototype chains.

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);
 
     var event = new NotificationEvent('NotificationEvent');
     assert_equals(event.type, 'NotificationEvent');
-    assert_own_property(event, 'notification');
+    assert_will_be_idl_attribute(event, 'notification');
     assert_equals(event.cancelable, false);
     assert_equals(event.bubbles, false);
     assert_equals(event.notification, null);
     assert_inherits(event, 'waitUntil');
 
     var notification = new Notification('foo');
     var eventWithInit = new NotificationEvent('NotificationEvent',
                                               { cancelable: true,
                                                 bubbles: true,
                                                 notification: notification,
                                               });
     assert_equals(eventWithInit.cancelable, true);
     assert_equals(eventWithInit.bubbles, true);
     assert_equals(eventWithInit.notification, notification);
 
 }, '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.');
+    assert_will_be_idl_attribute(self, 'onnotificationclick',
+                                 'The notificationclick event exists.');
+    assert_will_be_idl_attribute(self, 'onnotificationerror',
+                                 'The notificationerror event exists.');
 
 }, 'The notificationclick and notificationerror events exist on the global scope.');