Makes display roots be destroyed by client

ash/mus/window_manager.h

 // Copyright 2015 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 ASH_MUS_WINDOW_MANAGER_H_
 #define ASH_MUS_WINDOW_MANAGER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 119 matching lines @@
   bool OnWmSetProperty(
       ui::Window* window,
       const std::string& name,
       std::unique_ptr<std::vector<uint8_t>>* new_data) override;
   ui::Window* OnWmCreateTopLevelWindow(
       std::map<std::string, std::vector<uint8_t>>* properties) override;
   void OnWmClientJankinessChanged(const std::set<ui::Window*>& client_windows,
                                   bool not_responding) override;
   void OnWmNewDisplay(ui::Window* window,
                       const display::Display& display) override;
+  void OnWmDisplayRemoved(ui::Window* window) override;
   void OnWmPerformMoveLoop(ui::Window* window,
                            ui::mojom::MoveLoopSource source,
                            const gfx::Point& cursor_location,
                            const base::Callback<void(bool)>& on_done) override;
   void OnWmCancelMoveLoop(ui::Window* window) override;
   ui::mojom::EventResult OnAccelerator(uint32_t id,
                                        const ui::Event& event) override;
 
   shell::Connector* connector_;
 
@@ skipping 19 matching lines @@
   std::map<uint16_t, AcceleratorHandler*> accelerator_handlers_;
   uint16_t next_accelerator_namespace_id_ = 0u;
 
   DISALLOW_COPY_AND_ASSIGN(WindowManager);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_WINDOW_MANAGER_H_