Introduce toSVGInlineFlowBox() and use it

Source/core/rendering/svg/SVGInlineFlowBox.h

 /*
  * Copyright (C) 2006 Oliver Hunt <ojh16@student.canterbury.ac.nz>
  * Copyright (C) 2006 Apple Computer Inc.
  *
  * 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 32 matching lines @@
     virtual void paint(PaintInfo&, const LayoutPoint&, LayoutUnit lineTop, LayoutUnit lineBottom);
 
     virtual FloatRect calculateBoundaries() const;
 
     static void computeTextMatchMarkerRectForRenderer(RenderSVGInlineText*);
 
 private:
     float m_logicalHeight;
 };
 
+inline SVGInlineFlowBox* toSVGInlineFlowBox(InlineBox* box)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!box || box->isSVGInlineFlowBox());
+    return static_cast<SVGInlineFlowBox*>(box);
+}
+
 } // namespace WebCore
 
 #endif // SVGInlineFlowBox_h