Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/android/platform_window_android.h

 // Copyright 2013 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_ANDROID_PLATFORM_WINDOW_ANDROID_H_
 #define UI_PLATFORM_WINDOW_ANDROID_PLATFORM_WINDOW_ANDROID_H_
 
 #include "base/android/jni_weak_ref.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 49 matching lines @@
                 jint key_code,
                 jint unicode_character);
 
  private:
   void ReleaseWindow();
 
   // Overridden from 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;
@@ skipping 12 matching lines @@
   PlatformImeControllerAndroid platform_ime_controller_;
 
   base::WeakPtrFactory<PlatformWindowAndroid> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformWindowAndroid);
 };
 
 }  // namespace ui
 
 #endif  // UI_PLATFORM_WINDOW_ANDROID_PLATFORM_WINDOW_ANDROID_H_