Add toSVGGlyphRefElement(), and use it as much as possible

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 76 matching lines @@
 
     // These 2 variables are used to determine content model.
     bool fountFirstGlyphRef = false;
     bool foundFirstAltGlyphItem = false;
 
     for (Node* child = firstChild(); child; child = child->nextSibling()) {
         if (!foundFirstAltGlyphItem && child->hasTagName(SVGNames::glyphRefTag)) {
             fountFirstGlyphRef = true;
             String referredGlyphName;
 
-            if (static_cast<SVGGlyphRefElement*>(child)->hasValidGlyphElement(referredGlyphName))
+            if (toSVGGlyphRefElement(child)->hasValidGlyphElement(referredGlyphName))
                 glyphNames.append(referredGlyphName);
             else {
                 // As the spec says "If any of the referenced glyphs are unavailable,
                 // then the character(s) that are inside of the 'altGlyph' element are
                 // rendered as if there were not an 'altGlyph' element surrounding
                 // those characters.".
                 glyphNames.clear();
                 return false;
             }
         } else if (!fountFirstGlyphRef && child->hasTagName(SVGNames::altGlyphItemTag)) {
             foundFirstAltGlyphItem = true;
             Vector<String> 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())
                 return true;
         }
     }
     return !glyphNames.isEmpty();
 }
 
 }
 
 #endif