Reduce forward declarations in core/rendering

Source/core/rendering/RenderLayerStackingNodeIterator.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 24 matching lines @@
 
 namespace WebCore {
 
 enum ChildrenIteration {
     NegativeZOrderChildren = 1,
     NormalFlowChildren = 1 << 1,
     PositiveZOrderChildren = 1 << 2,
     AllChildren = NegativeZOrderChildren | NormalFlowChildren | PositiveZOrderChildren
 };
 
-class RenderLayer;
 class RenderLayerStackingNode;
 
 // This iterator walks the RenderLayerStackingNode lists in the following order:
 // NegativeZOrderChildren -> NormalFlowChildren -> PositiveZOrderChildren.
 class RenderLayerStackingNodeIterator {
     WTF_MAKE_NONCOPYABLE(RenderLayerStackingNodeIterator);
 public:
     RenderLayerStackingNodeIterator(const RenderLayerStackingNode& root, unsigned whichChildren)
         : m_root(root)
         , m_remainingChildren(whichChildren)
@@ skipping 27 matching lines @@
     void setIndexToLastItem();
 
     const RenderLayerStackingNode& m_root;
     unsigned m_remainingChildren;
     int m_index;
 };
 
 } // namespace WebCore
 
 #endif // RenderLayerStackingNodeIterator_h