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

Source/core/svg/SVGAltGlyphItemElement.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 34 matching lines @@
     // The ‘altGlyphItem’ element defines a candidate set of possible glyph substitutions.
     // The first ‘altGlyphItem’ element whose referenced glyphs are all available is chosen.
     // Its glyphs are rendered instead of the character(s) that are inside of the referencing
     // ‘altGlyph’ element.
     //
     // Here we fill glyphNames and return true only if all referenced glyphs are valid and
     // there is at least one glyph.
     for (Node* child = firstChild(); child; child = child->nextSibling()) {
         if (child->hasTagName(SVGNames::glyphRefTag)) {
             String referredGlyphName;
-            if (static_cast<SVGGlyphRefElement*>(child)->hasValidGlyphElement(referredGlyphName))
+            if (toSVGGlyphRefElement(child)->hasValidGlyphElement(referredGlyphName))
                 glyphNames.append(referredGlyphName);
             else {
                 glyphNames.clear();
                 return false;
             }
         }
     }
     return !glyphNames.isEmpty();
 }
 
 }
 
 #endif