Oilpan: fix build after r181604.

Source/core/testing/DictionaryTest.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.
 
 #include "config.h"
 #include "DictionaryTest.h"
 
 #include "core/testing/InternalDictionary.h"
 
 namespace blink {
@@ skipping 70 matching lines @@
     m_booleanMember = Nullable<bool>();
     m_doubleMember = Nullable<double>();
     m_stringMember = String();
     m_stringMemberWithDefault = String("Should not be returned");
     m_stringSequenceMember = Nullable<Vector<String> >();
     m_stringSequenceOrNullMember = Nullable<Vector<String> >();
     m_elementMember = nullptr;
     m_elementOrNullMember = nullptr;
 }
 
-void DictionaryTest::trace(Visitor*)
+void DictionaryTest::trace(Visitor* visitor)
 {
+    visitor->trace(m_elementMember);
+    visitor->trace(m_elementOrNullMember);
 }
 
 }