Enable the Sky compositor again

sky/compositor/surface_holder.cc

Index: sky/compositor/surface_holder.cc
diff --git a/sky/compositor/surface_holder.cc b/sky/compositor/surface_holder.cc
index 5288efa517e457246a78b7bc06e76c6020ac17e4..de9662209b5f2ebcca2f9dad5de07816020cb28a 100644
--- a/sky/compositor/surface_holder.cc
+++ b/sky/compositor/surface_holder.cc
@@ -28,10 +28,14 @@ SurfaceHolder::SurfaceHolder(Client* client, mojo::Shell* shell)
 }
 
 SurfaceHolder::~SurfaceHolder() {
-  if (surface_id_)
+  if (surface_ && surface_id_)
     surface_->DestroySurface(surface_id_.Clone());
 }
 
+bool SurfaceHolder::IsReadyForFrame() const {
+  return surface_;
+}
+
 void SurfaceHolder::SubmitFrame(mojo::FramePtr frame) {
   surface_->SubmitFrame(surface_id_.Clone(), frame.Pass());
 }