Reduce forward declarations in core/dom

Source/core/dom/TreeScopeStyleSheetCollection.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 25 matching lines @@
 #include "wtf/FastAllocBase.h"
 #include "wtf/HashMap.h"
 #include "wtf/ListHashSet.h"
 #include "wtf/RefPtr.h"
 #include "wtf/Vector.h"
 #include "wtf/text/WTFString.h"
 
 namespace WebCore {
 
 class ContainerNode;
-class DocumentStyleSheetCollector;
-class StyleEngine;
 class Node;
 class StyleSheetContents;
-class StyleSheetList;
 class StyleRuleFontFace;
 
 class TreeScopeStyleSheetCollection : public StyleSheetCollection {
 public:
     void addStyleSheetCandidateNode(Node*, bool createdByParser);
     void removeStyleSheetCandidateNode(Node*, ContainerNode* scopingNode);
     bool hasStyleSheetCandidateNodes() const { return !m_styleSheetCandidateNodes.isEmpty(); }
 
     bool usesRemUnits() const { return m_usesRemUnits; }
 
@@ skipping 46 matching lines @@
     bool m_usesRemUnits;
 
     DocumentOrderedList m_styleSheetCandidateNodes;
     StyleSheetScopingNodeList m_scopingNodesForStyleScoped;
 };
 
 }
 
 #endif