Add toSVGMissingGlyphElement(), and use it.

Source/core/svg/SVGMissingGlyphElement.h

 /*
  * Copyright (C) 2007 Eric Seidel <eric@webkit.org>
  *
  * 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 #ifndef SVGMissingGlyphElement_h
 #define SVGMissingGlyphElement_h
 
 #if ENABLE(SVG_FONTS)
+#include "SVGNames.h"
 #include "core/svg/SVGElement.h"
 
 namespace WebCore {
 
 class SVGMissingGlyphElement FINAL : public SVGElement {
 public:
     static PassRefPtr<SVGMissingGlyphElement> create(const QualifiedName&, Document*);
 
 private:
     SVGMissingGlyphElement(const QualifiedName&, Document*);
 
     virtual bool rendererIsNeeded(const NodeRenderingContext&) { return false; }
 };
 
+inline SVGMissingGlyphElement* toSVGMissingGlyphElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::missing_glyphTag));
+    return static_cast<SVGMissingGlyphElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif // ENABLE(SVG_FONTS)
 #endif