Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/x11/x11_window.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_PLATFORM_WINDOW_X11_X11_WINDOW_H_
 #define UI_PLATFORM_WINDOW_X11_X11_WINDOW_H_
 
 #include "base/macros.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/platform_window/x11/x11_window_base.h"
 #include "ui/platform_window/x11/x11_window_export.h"
 
 namespace ui {
 
 // PlatformWindow implementation for X11. PlatformEvents are XEvents.
 class X11_WINDOW_EXPORT X11Window : public X11WindowBase,
                                     public PlatformEventDispatcher {
  public:
   X11Window(PlatformWindowDelegate* delegate, const gfx::Rect& bounds);
   ~X11Window() override;
 
   // PlatformWindow:
+  void PrepareForShutdown() override;
   void SetCursor(PlatformCursor cursor) override;
 
  private:
   void ProcessXInput2Event(XEvent* xev);
 
   // PlatformEventDispatcher:
   bool CanDispatchEvent(const PlatformEvent& event) override;
   uint32_t DispatchEvent(const PlatformEvent& event) override;
 
   DISALLOW_COPY_AND_ASSIGN(X11Window);
 };
 
 }  // namespace ui
 
 #endif  // UI_PLATFORM_WINDOW_X11_X11_WINDOW_H_