Attach panel while dragging to bring it in front of other panels.

ash/wm/toplevel_window_event_handler.h

Index: ash/wm/toplevel_window_event_handler.h
diff --git a/ash/wm/toplevel_window_event_handler.h b/ash/wm/toplevel_window_event_handler.h
index 16b4032b2743e3d5882414cd37a2ae7b6f35809c..12402df846932bcee19042b45ebca8f4c1bc3ed2 100644
--- a/ash/wm/toplevel_window_event_handler.h
+++ b/ash/wm/toplevel_window_event_handler.h
@@ -37,6 +37,8 @@ class ASH_EXPORT ToplevelWindowEventHandler
   explicit ToplevelWindowEventHandler(aura::Window* owner);
   virtual ~ToplevelWindowEventHandler();
 
+  const aura::Window* owner() const { return owner_; }
+
   // Overridden from ui::EventHandler:
   virtual void OnKeyEvent(ui::KeyEvent* event) OVERRIDE;
   virtual void OnMouseEvent(ui::MouseEvent* event) OVERRIDE;
@@ -85,6 +87,9 @@ class ASH_EXPORT ToplevelWindowEventHandler
   // Invoked from ScopedWindowResizer if the window is destroyed.
   void ResizerWindowDestroyed();
 
+  // The container which this event handler is handling events on.
+  aura::Window* owner_;
+
   // Are we running a nested message loop from RunMoveLoop().
   bool in_move_loop_;