cc: Replace LayerIterator with iterator that walks layer list and effect tree

cc/layers/layer_list_iterator.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_LAYERS_LAYER_LIST_ITERATOR_H_
 #define CC_LAYERS_LAYER_LIST_ITERATOR_H_
 
 #include <stdlib.h>
 #include <vector>
 
 #include "cc/cc_export.h"
 
 namespace cc {
 
-class LayerImpl;
-
-// Unlike LayerIterator and friends, these iterators are not intended to permit
-// traversing the RSLL. Rather, they visit a collection of LayerImpls in
-// stacking order. All recursive walks over the LayerImpl tree should be
-// switched to use these classes instead as the concept of a LayerImpl tree is
-// deprecated.
+// This visits a tree of layers in drawing order. For LayerImpls, this is only
+// useful for tests, since there's no LayerImpl tree outside unit tests.
 template <typename LayerType>
 class CC_EXPORT LayerListIterator {
  public:
   explicit LayerListIterator(LayerType* root_layer);
   LayerListIterator(const LayerListIterator<LayerType>& other);
   virtual ~LayerListIterator();
 
   bool operator==(const LayerListIterator<LayerType>& other) const {
     return current_layer_ == other.current_layer_;
   }
@@ skipping 26 matching lines @@
 
  private:
   void DescendToRightmostInSubtree();
   LayerType* current_layer() { return this->current_layer_; }
   std::vector<size_t>& list_indices() { return this->list_indices_; }
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_LIST_ITERATOR_H_