Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/platform_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_PLATFORM_WINDOW_H_
 #define UI_PLATFORM_WINDOW_PLATFORM_WINDOW_H_
 
 #include <memory>
 
 #include "base/strings/string16.h"
@@ skipping 12 matching lines @@
 // Each instance of PlatformWindow represents a single window in the
 // underlying platform windowing system (i.e. X11/Win/OSX).
 class PlatformWindow {
  public:
   virtual ~PlatformWindow() {}
 
   virtual void Show() = 0;
   virtual void Hide() = 0;
   virtual void Close() = 0;
 
+  // Informs the window it is going to be destroyed sometime soon. This is only
+  // called for specific code paths, for example by Ash, so it shouldn't be
+  // assumed this will get called before destruction.
+  virtual void PrepareForShutdown() = 0;
+
   // Sets and gets the bounds of the platform-window. Note that the bounds is in
   // physical pixel coordinates.
   virtual void SetBounds(const gfx::Rect& bounds) = 0;
   virtual gfx::Rect GetBounds() = 0;
 
   virtual void SetTitle(const base::string16& title) = 0;
 
   virtual void SetCapture() = 0;
   virtual void ReleaseCapture() = 0;
 
@@ skipping 12 matching lines @@
   virtual void ConfineCursorToBounds(const gfx::Rect& bounds) = 0;
 
   // The PlatformImeController is owned by the PlatformWindow, the ownership is
   // not transferred.
   virtual PlatformImeController* GetPlatformImeController() = 0;
 };
 
 }  // namespace ui
 
 #endif  // UI_PLATFORM_WINDOW_PLATFORM_WINDOW_H_