Add toSVGRootInlineBox(), and use it as much as we can

Source/core/rendering/svg/SVGRootInlineBox.h

 /*
  * Copyright (C) 2006 Oliver Hunt <ojh16@student.canterbury.ac.nz>
  * Copyright (C) 2006 Apple Computer Inc.
  * 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 44 matching lines @@
 private:
     void reorderValueLists(Vector<SVGTextLayoutAttributes*>&);
     void layoutCharactersInTextBoxes(InlineFlowBox*, SVGTextLayoutEngine&);
     void layoutChildBoxes(InlineFlowBox*, FloatRect* = 0);
     void layoutRootBox(const FloatRect&);
 
 private:
     float m_logicalHeight;
 };
 
+inline SVGRootInlineBox* toSVGRootInlineBox(RootInlineBox* box)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!box || box->isSVGRootInlineBox());
+    return static_cast<SVGRootInlineBox*>(box);
+}
+
 } // namespace WebCore
 
 #endif // SVGRootInlineBox_h