Fix AshWindowTreeHost shutdown for Ozone X11.

ui/ozone/platform/cast/platform_window_cast.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 UI_OZONE_PLATFORM_CAST_PLATFORM_WINDOW_CAST_H_
 #define UI_OZONE_PLATFORM_CAST_PLATFORM_WINDOW_CAST_H_
 
 #include "base/macros.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/gfx/geometry/rect.h"
@@ skipping 10 matching lines @@
   PlatformWindowCast(PlatformWindowDelegate* delegate, const gfx::Rect& bounds);
   ~PlatformWindowCast() override;
 
   // PlatformWindow implementation:
   gfx::Rect GetBounds() override;
   void SetBounds(const gfx::Rect& bounds) override;
   void SetTitle(const base::string16& title) override;
   void Show() override {}
   void Hide() override {}
   void Close() override {}
+  void PrepareForShutdown() 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;
 
   // PlatformEventDispatcher implementation:
   bool CanDispatchEvent(const PlatformEvent& event) override;
   uint32_t DispatchEvent(const PlatformEvent& event) override;
 
  private:
   PlatformWindowDelegate* delegate_;
   gfx::Rect bounds_;
   gfx::AcceleratedWidget widget_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformWindowCast);
 };
 
 }  // namespace ui
 
 #endif  // UI_OZONE_PLATFORM_CAST_PLATFORM_WINDOW_CAST_H_