Setup LayerTree class, refactor 2 functions from LayerTreeHost to it.

cc/layers/layer_proto_converter.cc

Index: cc/layers/layer_proto_converter.cc
diff --git a/cc/layers/layer_proto_converter.cc b/cc/layers/layer_proto_converter.cc
index 25dd803c1ef17aece080cef7b4e831517b522ab8..2545783260cab6165aca5d05d4048b20dc051d48 100644
--- a/cc/layers/layer_proto_converter.cc
+++ b/cc/layers/layer_proto_converter.cc
@@ -57,9 +57,9 @@ void LayerProtoConverter::SerializeLayerProperties(
     LayerTreeHost* host,
     proto::LayerUpdate* layer_update) {
   TRACE_EVENT0("cc.remote", "LayerProtoConverter::SerializeLayerProperties");
-  for (auto layer : host->LayersThatShouldPushProperties())
+  for (auto layer : host->GetLayerTree()->LayersThatShouldPushProperties())
     layer->ToLayerPropertiesProto(layer_update);
-  host->LayersThatShouldPushProperties().clear();
+  host->GetLayerTree()->LayersThatShouldPushProperties().clear();
 }
 
 // static