Add support for (de)serializing LayerTreeHost.

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 6ac4cd38e7be7a32e57d72011cd862c3cf0375aa..c05bded9bc246376828b167a3e3d43e3de83afdb 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -35,6 +35,7 @@
 #include "cc/layers/layer_iterator.h"
 #include "cc/layers/layer_settings.h"
 #include "cc/layers/painted_scrollbar_layer.h"
+#include "cc/proto/layer_tree_host.pb.h"
 #include "cc/resources/ui_resource_request.h"
 #include "cc/scheduler/begin_frame_source.h"
 #include "cc/trees/draw_property_utils.h"
@@ -1269,4 +1270,8 @@ bool LayerTreeHost::IsThreaded() const {
   return compositor_mode_ == CompositorMode::Threaded;
 }
 
+void LayerTreeHost::ToProtobufForCommit(proto::LayerTreeHost* proto) const {}
+
+void LayerTreeHost::FromProtobufForCommit(const proto::LayerTreeHost& proto) {}
+
 }  // namespace cc