Clean up virtual functions in css/

Source/core/css/CSSUnknownRule.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 14 matching lines @@
 #include "core/css/CSSRule.h"
 
 namespace blink {
 
 class CSSUnknownRule final : public CSSRule {
     DEFINE_WRAPPERTYPEINFO();
 public:
     CSSUnknownRule() : CSSRule(0) { }
     virtual ~CSSUnknownRule() { }
 
-    virtual CSSRule::Type type() const override { return UNKNOWN_RULE; }
     virtual String cssText() const override { return String(); }
     virtual void reattach(StyleRuleBase*) override { }
     virtual void trace(Visitor* visitor) override { CSSRule::trace(visitor); }
+
+private:
+    virtual CSSRule::Type type() const override { return UNKNOWN_RULE; }
 };
 
 } // namespace blink
 
 #endif // CSSUnknownRule_h