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

Source/core/svg/SVGAltGlyphDefElement.cpp

 /*
  * 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 91 matching lines @@
                 // those characters.".
                 glyphNames.clear();
                 return false;
             }
         } else if (!fountFirstGlyphRef && child->hasTagName(SVGNames::altGlyphItemTag)) {
             foundFirstAltGlyphItem = true;
             Vector<AtomicString> referredGlyphNames;
 
             // As the spec says "The first 'altGlyphItem' in which all referenced glyphs
             // are available is chosen."
-            if (static_cast<SVGAltGlyphItemElement*>(child)->hasValidGlyphElements(glyphNames) && !glyphNames.isEmpty())
+            if (toSVGAltGlyphItemElement(child)->hasValidGlyphElements(glyphNames) && !glyphNames.isEmpty())
                 return true;
         }
     }
     return !glyphNames.isEmpty();
 }
 
 }
 
 #endif