bindings: Fixes layouttests when moving attributes to prototype chains.

LayoutTests/http/tests/push_messaging/subscribe_success.html

Index: LayoutTests/http/tests/push_messaging/subscribe_success.html
diff --git a/LayoutTests/http/tests/push_messaging/subscribe_success.html b/LayoutTests/http/tests/push_messaging/subscribe_success.html
index 196e6c4803369a3ba095fc47d46425643954b945..574a5c0590dcd2f6809cace4621a363eac7bcf66 100644
--- a/LayoutTests/http/tests/push_messaging/subscribe_success.html
+++ b/LayoutTests/http/tests/push_messaging/subscribe_success.html
@@ -25,10 +25,10 @@ async_test(function(test) {
             return swRegistration.pushManager.subscribe();
         })
         .then(function(pushSubscription) {
-            assert_own_property(pushSubscription, 'subscriptionId');
+            assert_will_be_idl_attribute(pushSubscription, 'subscriptionId');
             assert_equals(typeof pushSubscription.subscriptionId, 'string');
 
-            assert_own_property(pushSubscription, 'endpoint');
+            assert_will_be_idl_attribute(pushSubscription, 'endpoint');
             assert_equals(typeof pushSubscription.endpoint, 'string');
             try {
               var endpointUrl = new URL(pushSubscription.endpoint);