ui: Cleanup class/struct forward declarations

ui/views/widget/desktop_aura/desktop_drag_drop_client_aurax11.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_AURAX11_H_
 #define UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_DRAG_DROP_CLIENT_AURAX11_H_
 
 #include <X11/Xlib.h>
 
 #include <memory>
@@ skipping 19 matching lines @@
 class DragDropDelegate;
 }
 }
 
 namespace gfx {
 class ImageSkia;
 class Point;
 }
 
 namespace ui {
-class DragSource;
 class DropTargetEvent;
 class OSExchangeData;
 class OSExchangeDataProviderAuraX11;
 class SelectionFormatMap;
 }
 
 namespace views {
 class DesktopNativeCursorManager;
 class Widget;
 class X11MoveLoop;
@@ skipping 236 matching lines @@
   gfx::Vector2d drag_widget_offset_;
 
   base::WeakPtrFactory<DesktopDragDropClientAuraX11> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopDragDropClientAuraX11);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_DRAG_DROP_CLIENT_AURAX11_H_