cc: Add a main thread LayerListIterator

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/base/cc_export.h"
 
 namespace cc {
 
+class Layer;
 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.
+template <typename LayerType>
 class CC_EXPORT LayerListIterator {
  public:
-  explicit LayerListIterator(LayerImpl* root_layer);
-  LayerListIterator(const LayerListIterator& other);
+  explicit LayerListIterator(LayerType* root_layer);
+  LayerListIterator(const LayerListIterator<LayerType>& other);
   virtual ~LayerListIterator();
 
-  bool operator==(const LayerListIterator& other) const {
+  bool operator==(const LayerListIterator<LayerType>& other) const {
     return current_layer_ == other.current_layer_;
   }
 
-  bool operator!=(const LayerListIterator& other) const {
+  bool operator!=(const LayerListIterator<LayerType>& other) const {
     return !(*this == other);
   }
 
   // We will only support prefix increment.
   virtual LayerListIterator& operator++();
-  LayerImpl* operator->() const { return current_layer_; }
-  LayerImpl* operator*() const { return current_layer_; }
+  LayerType* operator->() const { return current_layer_; }
+  LayerType* operator*() const { return current_layer_; }
 
  protected:
   // The implementation of this iterator is currently tied tightly to the layer
   // tree, but it should be straightforward to reimplement in terms of a list
   // when it's ready.
-  LayerImpl* current_layer_;
+  LayerType* current_layer_;
   std::vector<size_t> list_indices_;
 };
 
-class CC_EXPORT LayerListReverseIterator : public LayerListIterator {
+template <typename LayerType>
+class CC_EXPORT LayerListReverseIterator : public LayerListIterator<LayerType> {
  public:
-  explicit LayerListReverseIterator(LayerImpl* root_layer);
+  explicit LayerListReverseIterator(LayerType* root_layer);
   ~LayerListReverseIterator() override;
 
   // We will only support prefix increment.
-  LayerListIterator& operator++() override;
+  LayerListIterator<LayerType>& operator++() override;
 
  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_