Introduce a SVGUnknownElement class for unknown SVG elements

Source/core/svg/SVGFEMergeNodeElement.h

 /*
  * Copyright (C) 2004, 2005 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006 Rob Buis <buis@kde.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,
@@ skipping 19 matching lines @@
 public:
     static PassRefPtr<SVGFEMergeNodeElement> create(const QualifiedName&, Document*);
 
 private:
     SVGFEMergeNodeElement(const QualifiedName&, Document*);
 
     bool isSupportedAttribute(const QualifiedName&);
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) OVERRIDE;
     virtual void svgAttributeChanged(const QualifiedName&);
 
+    virtual bool rendererIsNeeded(const NodeRenderingContext&) OVERRIDE { return false; }
+
     BEGIN_DECLARE_ANIMATED_PROPERTIES(SVGFEMergeNodeElement)
         DECLARE_ANIMATED_STRING(In1, in1)
     END_DECLARE_ANIMATED_PROPERTIES
 };
 
 } // namespace WebCore
 
 #endif