Add LayerTreeImpl end/begin/rend/rbegin

cc/trees/layer_tree_impl.h

 // Copyright 2012 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_TREES_LAYER_TREE_IMPL_H_
 #define CC_TREES_LAYER_TREE_IMPL_H_
 
 #include <map>
 #include <set>
 #include <string>
 #include <unordered_map>
 #include <vector>
 
 #include "base/macros.h"
 #include "base/values.h"
 #include "cc/base/synced_property.h"
 #include "cc/input/event_listener_properties.h"
 #include "cc/input/layer_selection_bound.h"
 #include "cc/layers/layer_impl.h"
+#include "cc/layers/layer_list_iterator.h"
 #include "cc/output/begin_frame_args.h"
 #include "cc/output/renderer.h"
 #include "cc/output/swap_promise.h"
 #include "cc/resources/ui_resource_client.h"
 #include "cc/trees/layer_tree_host_impl.h"
 #include "cc/trees/property_tree.h"
 
 namespace base {
 namespace trace_event {
 class TracedValue;
@@ skipping 105 matching lines @@
   void SetPropertyTrees(const PropertyTrees& property_trees) {
     property_trees_ = property_trees;
     property_trees_.transform_tree.set_source_to_parent_updates_allowed(false);
   }
   PropertyTrees* property_trees() { return &property_trees_; }
 
   void UpdatePropertyTreesForBoundsDelta();
 
   void PushPropertiesTo(LayerTreeImpl* tree_impl);
 
+  LayerListIterator begin();

[danakj, 2016/03/03 00:35:50]

Can you leave a comment saying what these iterate and in what order?

+  LayerListIterator end();
+  LayerListReverseIterator rbegin();
+  LayerListReverseIterator rend();
+
   // TODO(thakis): Consider marking this CC_EXPORT once we understand
   // http://crbug.com/575700 better.
   struct ElementLayers {
     // Transform and opacity mutations apply to this layer.
     LayerImpl* main = nullptr;
     // Scroll mutations apply to this layer.
     LayerImpl* scroll = nullptr;
   };
 
   void AddToElementMap(LayerImpl* layer);
@@ skipping 403 matching lines @@
 
   scoped_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_