Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/stub/stub_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_STUB_STUB_WINDOW_H_
 #define UI_PLATFORM_WINDOW_STUB_STUB_WINDOW_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/platform_window/platform_window.h"
 #include "ui/platform_window/stub/stub_window_export.h"
 
 namespace ui {
 
 class PlatformWindowDelegate;
 
 class STUB_WINDOW_EXPORT StubWindow : NON_EXPORTED_BASE(public PlatformWindow) {
  public:
   StubWindow(PlatformWindowDelegate* delegate,
              bool use_default_accelerated_widget = true);
   ~StubWindow() override;
 
  private:
   // PlatformWindow:
   void Show() override;
   void Hide() override;
   void Close() override;
+  void PrepareForShutdown() override;
   void SetBounds(const gfx::Rect& bounds) override;
   gfx::Rect GetBounds() override;
   void SetTitle(const base::string16& title) override;
   void SetCapture() override;
   void ReleaseCapture() override;
   void ToggleFullscreen() override;
   void Maximize() override;
   void Minimize() override;
   void Restore() override;
   void SetCursor(PlatformCursor cursor) override;
   void MoveCursorTo(const gfx::Point& location) override;
   void ConfineCursorToBounds(const gfx::Rect& bounds) override;
   PlatformImeController* GetPlatformImeController() override;
 
   PlatformWindowDelegate* delegate_;
   gfx::Rect bounds_;
 
   DISALLOW_COPY_AND_ASSIGN(StubWindow);
 };
 
 }  // namespace ui
 
 #endif  // UI_PLATFORM_WINDOW_STUB_STUB_WINDOW_H_