Add font support to PageSerializer

Source/core/css/CSSStyleRule.h

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * (C) 2002-2003 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2002, 2006, 2008, 2012 Apple Inc. 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.
  *
@@ skipping 39 matching lines @@
 
 private:
     CSSStyleRule(StyleRule*, CSSStyleSheet*);
 
     String generateSelectorText() const;
 
     RefPtr<StyleRule> m_styleRule;
     mutable RefPtr<StyleRuleCSSStyleDeclaration> m_propertiesCSSOMWrapper;
 };
 
+inline CSSStyleRule* toCSSStyleRule(CSSRule* rule)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(rule->type() == CSSRule::STYLE_RULE);
+    return static_cast<CSSStyleRule*>(rule);
+}
+
 } // namespace WebCore
 
 #endif // CSSStyleRule_h