Oilpan: move HistoryItem to the heap.

Source/core/loader/HistoryItem.cpp

 /*
  * Copyright (C) 2005, 2006, 2008, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 196 matching lines @@
 {
     return m_formData.get();
 }
 
 bool HistoryItem::isCurrentDocument(Document* doc) const
 {
     // FIXME: We should find a better way to check if this is the current document.
     return equalIgnoringFragmentIdentifier(url(), doc->url());
 }
 
+void HistoryItem::trace(Visitor* visitor)
+{
+    visitor->trace(m_documentState);
+}
+
 } // namespace blink
-