Allow Oilpan heap objects account for their external allocations.

Source/core/dom/Text.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. 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 43 matching lines @@
     bool textRendererIsNeeded(const LayoutStyle&, const LayoutObject& parent);
     RenderText* createTextRenderer(const LayoutStyle&);
     void updateTextRenderer(unsigned offsetOfReplacedData, unsigned lengthOfReplacedData, RecalcStyleBehavior = DoNotRecalcStyle);
 
     virtual void attach(const AttachContext& = AttachContext()) override final;
     void reattachIfNeeded(const AttachContext& = AttachContext());
 
     virtual bool canContainRangeEndPoint() const override final { return true; }
     virtual NodeType nodeType() const override;
 
+    virtual void trace(Visitor*) override;
+
 protected:
     Text(TreeScope& treeScope, const String& data, ConstructionType type)
         : CharacterData(treeScope, data, type) { }
 
 private:
     virtual String nodeName() const override;
     virtual PassRefPtrWillBeRawPtr<Node> cloneNode(bool deep = true) override final;
 
     bool isTextNode() const = delete; // This will catch anyone doing an unnecessary check.
 
     bool needsWhitespaceRenderer();
 
     virtual PassRefPtrWillBeRawPtr<Text> cloneWithData(const String&);
 
 #ifndef NDEBUG
     virtual void formatForDebugger(char* buffer, unsigned length) const override;
 #endif
 };
 
 DEFINE_NODE_TYPE_CASTS(Text, isTextNode());
 
 } // namespace blink
 
 #endif // Text_h