bindings: Fixes push_messaging/interfaces.html test.

LayoutTests/http/tests/push_messaging/resources/interfaces-worker.js

 importScripts('/resources/testharness.js');
 importScripts('/resources/testharness-helpers.js');
 
 test(function() {
     assert_own_property(self, 'PushEvent');
 
     var event = new PushEvent('PushEvent');
     assert_will_be_idl_attribute(event, 'data');
     assert_equals(event.type, 'PushEvent');
 
     // PushEvent should be extending ExtendableEvent.
     assert_inherits(event, 'waitUntil');
 
 }, 'PushEvent should be exposed and have the expected interface.');
 
 test(function() {
     assert_own_property(self, 'PushManager', 'PushManager needs to be exposed as a global.');
-    assert_own_property(registration, 'pushManager', 'PushManager needs to be exposed on the registration.');
+    assert_will_be_idl_attribute(registration, 'pushManager', 'PushManager needs to be exposed on the registration.');
 
     assert_inherits(registration.pushManager, 'subscribe');
     assert_inherits(registration.pushManager, 'getSubscription');
     assert_inherits(registration.pushManager, 'hasPermission');
 
 }, 'PushManager should be exposed and have the expected interface.');
 
 test(function() {
     assert_own_property(self, 'PushMessageData', 'PushMessageData needs to be exposed as a global.');
 
     var pushMessageData = new PushMessageData('SomeData');
     assert_inherits(pushMessageData, 'arrayBuffer');
     assert_inherits(pushMessageData, 'blob');
     assert_inherits(pushMessageData, 'json');
     assert_inherits(pushMessageData, 'text');
 
 }, 'PushMessageData should be exposed and have the expected interface.');
 
 test(function() {
     assert_own_property(self, 'PushSubscription', 'PushSubscription needs to be exposed as a global.');
 
     // FIXME: Assert existence of the attributes when they are properly
     // exposed in the prototype chain. https://crbug.com/43394
 
     assert_own_property(PushSubscription.prototype, 'unsubscribe');
 
 }, 'PushSubscription should be exposed and have the expected interface.');