Make reftests work for sky.

sky/compositor/layer_host.cc

Index: sky/compositor/layer_host.cc
diff --git a/sky/compositor/layer_host.cc b/sky/compositor/layer_host.cc
index 36b3ec26e5cc34556cfd0dc6baee6cc7b3e65137..9c47732a4057a4e09b7ed64a0b2c4c4b88df022f 100644
--- a/sky/compositor/layer_host.cc
+++ b/sky/compositor/layer_host.cc
@@ -42,6 +42,10 @@ void LayerHost::SetRootLayer(scoped_refptr<Layer> layer) {
   root_layer_->set_host(this);
 }
 
+std::string LayerHost::GetPixels() {
+  return root_layer_->GetPixels();
+}
+
 void LayerHost::OnSurfaceConnectionCreated() {
   DCHECK_EQ(state_, kWaitingForSurfaceService);
   state_ = kReadyForFrame;