mus: aura::Env should be destroyed last in the mus client.

ui/aura/mus/focus_synchronizer.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 UI_AURA_MUS_FOCUS_SYNCHRONIZER_H_
 #define UI_AURA_MUS_FOCUS_SYNCHRONIZER_H_
 
 #include "base/macros.h"
 #include "ui/aura/client/focus_change_observer.h"
 #include "ui/aura/env_observer.h"
@@ skipping 35 matching lines @@
   void SetActiveFocusClient(client::FocusClient* focus_client);
 
   // Called internally to set |focused_window_| and update the server.
   void SetFocusedWindow(WindowMus* window);
 
   // Overriden from client::FocusChangeObserver:
   void OnWindowFocused(Window* gained_focus, Window* lost_focus) override;
 
   // Overrided from EnvObserver:
   void OnWindowInitialized(Window* window) override;
-  void OnWillDestroyEnv() override;
   void OnActiveFocusClientChanged(client::FocusClient* focus_client,
                                   Window* window) override;
 
   FocusSynchronizerDelegate* delegate_;
   ui::mojom::WindowTree* window_tree_;
 
   client::FocusClient* active_focus_client_ = nullptr;
 
   bool setting_focus_ = false;
   WindowMus* window_setting_focus_to_ = nullptr;
   WindowMus* focused_window_ = nullptr;
 
   // Cached so we can detect if Env is destroyed before us.
   Env* env_;
 
   DISALLOW_COPY_AND_ASSIGN(FocusSynchronizer);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_FOCUS_SYNCHRONIZER_H_