aura-mus: add a drag representation image.

services/ui/ws/drag_source.h

Index: services/ui/ws/drag_source.h
diff --git a/services/ui/ws/drag_source.h b/services/ui/ws/drag_source.h
index 81428d04341938d34af24abfbb4bc8f33dfccf3a..050a0ced79439ad49458eed5aa637f09842d1c7b 100644
--- a/services/ui/ws/drag_source.h
+++ b/services/ui/ws/drag_source.h
@@ -18,6 +18,9 @@ class DragSource {
  public:
   virtual ~DragSource() {}
 
+  // Called during a drag operation when the mouse cursor moves.
+  virtual void OnDragMoved(const gfx::Point& location) = 0;
+
   // Called when a drag operation is completed. |success| is true when a target
   // window signaled the successful completion of the drag, false in all other
   // cases where a drag was aborted at any step in the process. |action_taken|