Fix crash when the source browser window is deleted during a drag

ui/views/widget/desktop_aura/desktop_drag_drop_client_win.h

 // Copyright (c) 2012 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_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_DRAG_DROP_CLIENT_WIN_H_
 #define UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_DRAG_DROP_CLIENT_WIN_H_
 
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
 #include "ui/views/views_export.h"
 #include "ui/wm/public/drag_drop_client.h"
 
 namespace ui {
 class DragSourceWin;
 }
 
 namespace views {
 class DesktopDropTargetWin;
 
@@ skipping 22 matching lines @@
 
  private:
   bool drag_drop_in_progress_;
 
   int drag_operation_;
 
   scoped_refptr<ui::DragSourceWin> drag_source_;
 
   scoped_refptr<DesktopDropTargetWin> drop_target_;
 
+  base::WeakPtrFactory<DesktopDragDropClientWin> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(DesktopDragDropClientWin);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_DRAG_DROP_CLIENT_WIN_H_