Add support for (de)serializing LayerTreeHost.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 2dc93ee2bffcb5a45843584253e0d5e988298253..1a66e9117a0dc4cec726c16baa32ae0aa4146d71 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -69,6 +69,10 @@ struct PendingPageScaleAnimation;
 struct RenderingStats;
 struct ScrollAndScaleSet;
 
+namespace proto {
+class LayerTreeHost;
+}
+
 class CC_EXPORT LayerTreeHost : public MutatorHostClient {
  public:
   // TODO(sad): InitParams should be a movable type so that it can be
@@ -363,6 +367,9 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
   bool HasAnyAnimation(const Layer* layer) const;
   bool HasActiveAnimation(const Layer* layer) const;
 
+  void ToProtobufForCommit(proto::LayerTreeHost* proto) const;
+  void FromProtobufForCommit(const proto::LayerTreeHost& proto);
+
  protected:
   LayerTreeHost(InitParams* params, CompositorMode mode);
   void InitializeThreaded(