IDL: Fix build bugs in IDL dictionary impl generation

Source/bindings/tests/results/core/TestDictionary.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file has been auto-generated by code_generator_v8.py. DO NOT MODIFY!
 
 #include "config.h"
 #include "TestDictionary.h"
 
 
 namespace blink {
 
 TestDictionary::TestDictionary()
 {
     setLongMember(1);
     setStringOrNullMember(String("default string value"));
     setEnumMember(String("foo"));
 }
 
 void TestDictionary::trace(Visitor* visitor)
 {
     visitor->trace(m_testInterfaceGarbageCollectedMember);
     visitor->trace(m_testInterfaceGarbageCollectedOrNullMember);
     visitor->trace(m_testInterfaceWillBeGarbageCollectedMember);
     visitor->trace(m_testInterfaceWillBeGarbageCollectedOrNullMember);
+    visitor->trace(m_elementOrNullMember);
 }
 
 } // namespace blink