Reduce forward declarations in core/dom

Source/core/dom/Comment.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2009 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.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #ifndef Comment_h
 #define Comment_h
 
 #include "core/dom/CharacterData.h"
 
 namespace WebCore {
 
-class ExecutionContext;
-
 class Comment FINAL : public CharacterData {
 public:
     static PassRefPtrWillBeRawPtr<Comment> create(Document&, const String&);
 
 private:
     Comment(Document&, const String&);
 
     virtual String nodeName() const OVERRIDE;
     virtual NodeType nodeType() const OVERRIDE;
     virtual PassRefPtrWillBeRawPtr<Node> cloneNode(bool deep = true) OVERRIDE;
 };
 
 DEFINE_NODE_TYPE_CASTS(Comment, nodeType() == Node::COMMENT_NODE);
 
 } // namespace WebCore
 
 #endif // Comment_h