cc: Add a main thread LayerListIterator

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 17b1e6b8301ff32ad5edb80b4d1a7a7e668f03e8..47691bf9e2b47ae6f6ba51f0a1f1679f3a334710 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -902,6 +902,22 @@ bool LayerTreeHost::UpdateLayers() {
   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;
 }