Clean up static_cast<SVGFooElement*> by using toSVGFoo()

Source/core/svg/SVGAltGlyphElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2007 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006 Rob Buis <buis@kde.org>
  * Copyright (C) 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Torch Mobile (Beijing) Co. Ltd. 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 81 matching lines @@
     Element* element = targetElementFromIRIString(getAttribute(XLinkNames::hrefAttr), document(), &target);
     if (!element)
         return false;
 
     if (element->hasTagName(SVGNames::glyphTag)) {
         glyphNames.append(target);
         return true;
     }
 
     if (element->hasTagName(SVGNames::altGlyphDefTag)
-        && static_cast<SVGAltGlyphDefElement*>(element)->hasValidGlyphElements(glyphNames))
+        && toSVGAltGlyphDefElement(element)->hasValidGlyphElements(glyphNames))
         return true;
 
     return false;
 }
 
 }
 
 #endif