Introduce a SVGUnknownElement class for unknown SVG elements

Source/core/svg/SVGAltGlyphDefElement.h

 /*
  * Copyright (C) 2011 Leo Yang <leoyang@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
@@ skipping 16 matching lines @@
 namespace WebCore {
 
 class SVGAltGlyphDefElement FINAL : public SVGElement {
 public:
     static PassRefPtr<SVGAltGlyphDefElement> create(const QualifiedName&, Document*);
 
     bool hasValidGlyphElements(Vector<String>& glyphNames) const;
 
 private:
     SVGAltGlyphDefElement(const QualifiedName&, Document*);
+
+    virtual bool rendererIsNeeded(const NodeRenderingContext&) OVERRIDE { return false; }
 };
 
 }
 
 #endif
 #endif