Generalize layer mirroring for phantom windows

ui/compositor/layer_tree_owner.h

Index: ui/compositor/layer_tree_owner.h
diff --git a/ui/compositor/layer_tree_owner.h b/ui/compositor/layer_tree_owner.h
index a54142bef31d75416d4daf0287079925936ec678..4412c2225a09d216fa9b2a57e8adf723248ee965 100644
--- a/ui/compositor/layer_tree_owner.h
+++ b/ui/compositor/layer_tree_owner.h
@@ -5,6 +5,8 @@
 #ifndef UI_COMPOSITOR_LAYER_TREE_OWNER_H_
 #define UI_COMPOSITOR_LAYER_TREE_OWNER_H_
 
+#include <memory>
+
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "ui/compositor/compositor_export.h"
@@ -16,7 +18,7 @@ class Layer;
 // Scoping object that owns a Layer and all its descendants.
 class COMPOSITOR_EXPORT LayerTreeOwner {
  public:
-  explicit LayerTreeOwner(Layer* root);
+  explicit LayerTreeOwner(std::unique_ptr<Layer> root);
   ~LayerTreeOwner();
 
   Layer* release() WARN_UNUSED_RESULT {