Generalize layer mirroring for phantom windows

ui/compositor/layer_tree_owner.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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"
 
 namespace ui {
 
 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 {
     Layer* root = root_;
     root_ = NULL;
     return root;
   }
 
   Layer* root() { return root_; }
   const Layer* root() const { return root_; }
 
  private:
   Layer* root_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeOwner);
 };
 
 }  // namespace
 
 #endif  // UI_COMPOSITOR_LAYER_TREE_OWNER_H_