cc: Add a main thread LayerListIterator

cc/trees/layer_tree_host.cc

 // Copyright 2011 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.
 
 #include "cc/trees/layer_tree_host.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 884 matching lines @@
 bool LayerTreeHost::UpdateLayers() {
   DCHECK(!output_surface_lost_);
   if (!root_layer())
     return false;
   DCHECK(!root_layer()->parent());
   bool result = DoUpdateLayers(root_layer());
   micro_benchmark_controller_.DidUpdateLayers();
   return result || next_commit_forces_redraw_;
 }
 
+LayerListIterator<Layer> LayerTreeHost::begin() {
+  return LayerListIterator<Layer>(root_layer_.get());
+}
+
+LayerListIterator<Layer> LayerTreeHost::end() {
+  return LayerListIterator<Layer>(nullptr);
+}
+
+LayerListReverseIterator<Layer> LayerTreeHost::rbegin() {
+  return LayerListReverseIterator<Layer>(root_layer_.get());
+}
+
+LayerListReverseIterator<Layer> LayerTreeHost::rend() {
+  return LayerListReverseIterator<Layer>(nullptr);
+}
+
 void LayerTreeHost::DidCompletePageScaleAnimation() {
   did_complete_scale_animation_ = true;
 }
 
 static Layer* FindFirstScrollableLayer(Layer* layer) {
   if (!layer)
     return NULL;
 
   if (layer->scrollable())
     return layer;
@@ skipping 734 matching lines @@
       [seq_num](Layer* layer) {
         layer->set_property_tree_sequence_number(seq_num);
       },
       CallFunctionLayerType::ALL_LAYERS);
 
   surface_id_namespace_ = proto.surface_id_namespace();
   next_surface_sequence_ = proto.next_surface_sequence();
 }
 
 }  // namespace cc