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

cc/layers/layer_proto_converter.cc

 // Copyright 2015 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/layers/layer_proto_converter.h"
 
 #include "base/stl_util.h"
 #include "base/trace_event/trace_event.h"
 #include "cc/layers/empty_content_layer_client.h"
 #include "cc/layers/heads_up_display_layer.h"
@@ skipping 39 matching lines @@
   }
   new_root->FromLayerNodeProto(root_node, layer_id_map, layer_tree_host);
   return new_root;
 }
 
 // static
 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
 void LayerProtoConverter::DeserializeLayerProperties(
     Layer* existing_root,
     const proto::LayerUpdate& layer_update) {
   DCHECK(existing_root);
   LayerIdMap layer_id_map;
   RecursivelyFindAllLayers(existing_root, &layer_id_map);
 
@@ skipping 45 matching lines @@
       return SolidColorScrollbarLayer::Create(ScrollbarOrientation::HORIZONTAL,
                                               -1, -1, false, Layer::INVALID_ID);
   }
   // TODO(nyquist): Add the rest of the necessary LayerTypes. This function
   // should not return null.
   NOTREACHED();
   return nullptr;
 }
 
 }  // namespace cc