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

cc/proto/layer_tree_host.proto

 // 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.
 
 syntax = "proto2";
 
 import "display_item.proto";
 import "layer.proto";
 import "layer_selection_bound.proto";
+import "layer_tree.proto";
 import "layer_tree_debug_state.proto";
 import "property_tree.proto";
 import "size.proto";
 import "vector2df.proto";
 
 package cc.proto;
 
 option optimize_for = LITE_RUNTIME;
 
 message SkPictureData {
@@ skipping 38 matching lines @@
   optional int32 page_scale_layer_id = 27;
   optional int32 inner_viewport_scroll_layer_id = 28;
   optional int32 outer_viewport_scroll_layer_id = 29;
   optional LayerSelection selection = 30;
   optional PropertyTrees property_trees = 31;
   optional uint32 surface_client_id = 32;
   optional uint32 next_surface_sequence = 33;
   optional uint32 wheel_event_listener_properties = 34;
   optional bool have_scroll_event_handlers = 35;
   optional uint32 touch_start_or_move_event_listener_properties = 36;
-  repeated int32 layers_that_should_push_properties = 37;
+  optional LayerTree layer_tree = 37;
   optional uint32 touch_end_or_cancel_event_listener_properties = 38;
   optional SkPictures pictures = 39;
 }