Reduce forward declarations in core/dom

Source/core/dom/DOMImplementation.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2008 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 11 matching lines @@
  */
 
 #ifndef DOMImplementation_h
 #define DOMImplementation_h
 
 #include "core/dom/Document.h"
 #include "wtf/PassRefPtr.h"
 
 namespace WebCore {
 
-class CSSStyleSheet;
 class Document;
 class DocumentInit;
 class DocumentType;
 class ExceptionState;
 class LocalFrame;
 class HTMLDocument;
 class KURL;
 class XMLDocument;
 
 class DOMImplementation FINAL : public NoBaseWillBeGarbageCollectedFinalized<DOMImplementation>, public ScriptWrappable {
@@ skipping 31 matching lines @@
 
 private:
     explicit DOMImplementation(Document&);
 
     RawPtrWillBeMember<Document> m_document;
 };
 
 } // namespace WebCore
 
 #endif