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 c6efe95b720c21513460ab11ffe57b2797a83f60..423dbdb497c2bbcaac2155f060d01c3da5474058 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -898,6 +898,22 @@ 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;
 }