Oilpan: add missing HTMLFormControlsCollection tracing.

Source/core/html/HTMLFormControlsCollection.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Samsung Electronics. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 28 matching lines @@
 
 class HTMLFormControlsCollection FINAL : public HTMLCollection {
 public:
     static PassRefPtrWillBeRawPtr<HTMLFormControlsCollection> create(ContainerNode&, CollectionType);
 
     virtual ~HTMLFormControlsCollection();
 
     virtual Element* namedItem(const AtomicString& name) const OVERRIDE;
     void namedGetter(const AtomicString& name, bool& radioNodeListEnabled, RefPtrWillBeRawPtr<RadioNodeList>&, bool& elementEnabled, RefPtrWillBeRawPtr<Element>&);
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     explicit HTMLFormControlsCollection(ContainerNode&);
 
     virtual void updateIdNameCache() const OVERRIDE;
     virtual void supportedPropertyNames(Vector<String>& names) OVERRIDE;
 
     const FormAssociatedElement::List& formControlElements() const;
     const WillBeHeapVector<RawPtrWillBeMember<HTMLImageElement> >& formImageElements() const;
     virtual Element* virtualItemAfter(Element*) const OVERRIDE;
     virtual void invalidateCache(Document* oldDocument = 0) const OVERRIDE;
 
-    mutable Element* m_cachedElement;
+    mutable RawPtrWillBeMember<Element> m_cachedElement;
     mutable unsigned m_cachedElementOffsetInArray;
 };
 
 } // namespace
 
 #endif