Use tighter typing for RenderTextFragment::firstLetter()

Source/core/rendering/RenderTextFragment.h

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007 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 27 matching lines @@
     virtual ~RenderTextFragment();
 
     virtual bool isTextFragment() const OVERRIDE { return true; }
 
     virtual bool canBeSelectionLeaf() const OVERRIDE { return node() && node()->rendererIsEditable(); }
 
     unsigned start() const { return m_start; }
     unsigned end() const { return m_end; }
     virtual unsigned textStartOffset() const OVERRIDE { return start(); }
 
-    RenderObject* firstLetter() const { return m_firstLetter; }
-    void setFirstLetter(RenderObject* firstLetter) { m_firstLetter = firstLetter; }
+    RenderBoxModelObject* firstLetter() const { return m_firstLetter; }
+    void setFirstLetter(RenderBoxModelObject* firstLetter) { m_firstLetter = firstLetter; }
     RenderText* firstRenderTextInFirstLetter() const;
 
     StringImpl* contentString() const { return m_contentString.get(); }
     virtual PassRefPtr<StringImpl> originalText() const OVERRIDE;
 
     virtual void setText(PassRefPtr<StringImpl>, bool force = false) OVERRIDE;
 
     virtual void transformText() OVERRIDE;
 
 protected:
     virtual void styleDidChange(StyleDifference, const RenderStyle* oldStyle) OVERRIDE;
 
 private:
     virtual void willBeDestroyed() OVERRIDE;
 
     virtual UChar previousCharacter() const OVERRIDE;
     RenderBlock* blockForAccompanyingFirstLetter() const;
     virtual void updateHitTestResult(HitTestResult&, const LayoutPoint&) OVERRIDE;
 
     unsigned m_start;
     unsigned m_end;
     RefPtr<StringImpl> m_contentString;
-    RenderObject* m_firstLetter;
+    RenderBoxModelObject* m_firstLetter;
 };
 
 DEFINE_TYPE_CASTS(RenderTextFragment, RenderObject, object, toRenderText(object)->isTextFragment(), toRenderText(object).isTextFragment());
 
 } // namespace WebCore
 
 #endif // RenderTextFragment_h