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

cc/proto/layer_tree_host.proto

Index: cc/proto/layer_tree_host.proto
diff --git a/cc/proto/layer_tree_host.proto b/cc/proto/layer_tree_host.proto
index 71bbcf529dfcb3490ed9d3e42291b3618c02b792..452c63c1866061a71e95692763194de3488b7a3a 100644
--- a/cc/proto/layer_tree_host.proto
+++ b/cc/proto/layer_tree_host.proto
@@ -7,6 +7,7 @@ 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";
@@ -65,7 +66,7 @@ message LayerTreeHost {
   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;
 }