Introduce DEFINE_CUSTOM_FONT_DATA_TYPE_CASTS, and use it

Source/core/rendering/svg/SVGTextLayoutEngineSpacing.cpp

 /*
  * Copyright (C) Research In Motion Limited 2010. 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.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 39 matching lines @@
     const SimpleFontData* fontData = m_font.primaryFont();
     if (!fontData->isSVGFont()) {
         m_lastGlyph = 0;
         return 0;
     }
 
     ASSERT(fontData->isCustomFont());
     ASSERT(fontData->isSVGFont());
 
     RefPtr<CustomFontData> customFontData = fontData->customFontData();
-    const SVGFontData* svgFontData = static_cast<const SVGFontData*>(customFontData.get());
-    SVGFontFaceElement* svgFontFace = svgFontData->svgFontFaceElement();
+    SVGFontFaceElement* svgFontFace = toSVGFontData(customFontData)->svgFontFaceElement();
     ASSERT(svgFontFace);
 
     SVGFontElement* svgFont = svgFontFace->associatedFontElement();
     if (!svgFont) {
         m_lastGlyph = 0;
         return 0;
     }
 
     float kerning = 0;
     if (m_lastGlyph) {
@@ skipping 26 matching lines @@
             spacing += m_font.fontDescription().wordSpacing();
     }
 
     if (m_effectiveZoom != 1)
         spacing = spacing / m_effectiveZoom;
 
     return spacing;
 }
 
 }