Generalize layer mirroring for phantom windows

ui/compositor/layer_tree_owner.cc

 // 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.
 
 #include "ui/compositor/layer_tree_owner.h"
 
 #include "ui/compositor/layer.h"
 
 namespace ui {
 
 namespace {
 
 // Deletes |layer| and all its descendants.
 void DeepDeleteLayers(Layer* layer) {
   std::vector<Layer*> children = layer->children();
   for (std::vector<Layer*>::const_iterator it = children.begin();
        it != children.end();
        ++it) {
     Layer* child = *it;
     DeepDeleteLayers(child);
   }
   delete layer;
 }
 
 }  // namespace
 
-LayerTreeOwner::LayerTreeOwner(Layer* root) : root_(root) {}
+LayerTreeOwner::LayerTreeOwner(std::unique_ptr<Layer> root)
+    : root_(root.release()) {}
 
 LayerTreeOwner::~LayerTreeOwner() {
   if (root_)
     DeepDeleteLayers(root_);
 }
 
 }  // namespace ui