Add font support to PageSerializer

Source/core/css/CSSFontFaceSrcValue.h

 /*
  * Copyright (C) 2007, 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 77 matching lines @@
     String m_format;
     bool m_isLocal;
 
     ResourcePtr<FontResource> m_fetched;
 
 #if ENABLE(SVG_FONTS)
     SVGFontFaceElement* m_svgFontFaceElement;
 #endif
 };
 
+inline CSSFontFaceSrcValue* toCSSFontFaceSrcValue(CSSValue* value)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!value || value->isFontFaceSrcValue());
+    return static_cast<CSSFontFaceSrcValue*>(value);
+}
+
 }
 
 #endif