Inroduce toSVGInlineTextBox, and use it.

Source/core/rendering/svg/SVGInlineTextBox.h

 /*
  * Copyright (C) 2007 Rob Buis <buis@kde.org>
  * Copyright (C) 2007 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) Research In Motion Limited 2010. 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 70 matching lines @@
     virtual bool nodeAtPoint(const HitTestRequest&, HitTestResult&, const HitTestLocation& locationInContainer, const LayoutPoint& accumulatedOffset, LayoutUnit lineTop, LayoutUnit lineBottom) OVERRIDE;
 
 private:
     float m_logicalHeight;
     unsigned m_paintingResourceMode : 4;
     unsigned m_startsNewTextChunk : 1;
     RenderSVGResource* m_paintingResource;
     Vector<SVGTextFragment> m_textFragments;
 };
 
+inline SVGInlineTextBox* toSVGInlineTextBox(InlineBox* box)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!box || box->isSVGInlineTextBox());
+    return static_cast<SVGInlineTextBox*>(box);
+}
+
 } // namespace WebCore
 
 #endif // SVGInlineTextBox_h