Makes display roots be destroyed by client

services/ui/ws/window_manager_display_root.h

 // Copyright 2016 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 SERVICES_UI_WS_WINDOW_MANAGER_DISPLAY_ROOT_H_
 #define SERVICES_UI_WS_WINDOW_MANAGER_DISPLAY_ROOT_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 26 matching lines @@
   Display* display() { return display_; }
   const Display* display() const { return display_; }
 
   WindowManagerState* window_manager_state() { return window_manager_state_; }
   const WindowManagerState* window_manager_state() const {
     return window_manager_state_;
   }
 
  private:
   friend class Display;
+  friend class WindowManagerState;
 
   WindowServer* window_server();
 
   Display* display_;
   // Root ServerWindow of this WindowManagerDisplayRoot. |root_| has a parent,
   // the root ServerWindow of the Display.
   std::unique_ptr<ServerWindow> root_;
   WindowManagerState* window_manager_state_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(WindowManagerDisplayRoot);
 };
 
 }  // namespace ws
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_WINDOW_MANAGER_DISPLAY_ROOT_H_