Remove TreatNullAs=NullString for CSS*Rule

Source/core/css/CSSCharsetRule.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 24 matching lines @@
         return adoptRef(new CSSCharsetRule(parent, encoding));
     }
 
     virtual ~CSSCharsetRule() { }
 
     virtual CSSRule::Type type() const OVERRIDE { return CHARSET_RULE; }
     virtual String cssText() const OVERRIDE;
     virtual void reattach(StyleRuleBase* rule) OVERRIDE { ASSERT_UNUSED(rule, !rule); }
 
     const String& encoding() const { return m_encoding; }
-    void setEncoding(const String& encoding, ExceptionState&) { m_encoding = encoding; }
+    void setEncoding(const String& encoding) { m_encoding = encoding; }
 
 private:
     CSSCharsetRule(CSSStyleSheet* parent, const String& encoding);
 
     String m_encoding;
 };
 
 } // namespace WebCore
 
 #endif // CSSCharsetRule_h