Desktop aura: Break aura::Window::SetParent in two.

ash/drag_drop/drag_drop_tracker.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 ASH_DRAG_DROP_DRAG_DROP_TRACKER_H_
 #define ASH_DRAG_DROP_DRAG_DROP_TRACKER_H_
 
 #include "ash/ash_export.h"
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "ui/base/events/event.h"
 
 namespace aura {
+class RootWindow;
 class Window;
 }
 
 namespace ash {
 namespace internal {
 
 // Provides functions for handling drag events inside and outside the root
 // window where drag is started. This internally sets up a capture window for
 // tracking drag events outside the root window where drag is initiated.
 // ash/wm/coordinate_conversion.h is used internally and only X11 environment
 // is supported for now.
 class ASH_EXPORT DragDropTracker {
  public:
-  DragDropTracker();
+  explicit DragDropTracker(aura::RootWindow* context_root);
   ~DragDropTracker();
 
   aura::Window* capture_window() { return capture_window_.get(); }
 
   // Tells our |capture_window_| to take capture. This is not done right at
   // creation to give the caller a chance to perform any operations needed
   // before the capture is transfered.
   void TakeCapture();
 
   // Gets the target located at |event| in the coordinates of the active root
@@ skipping 10 matching lines @@
   // A window for capturing drag events while dragging.
   scoped_ptr<aura::Window> capture_window_;
 
   DISALLOW_COPY_AND_ASSIGN(DragDropTracker);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_DRAG_DROP_DRAG_DROP_TRACKER_H_