mus drag and drop: return the completed effect to the caller.

services/ui/ws/drag_source.h

 // Copyright 2016 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 SERVICES_UI_WS_DRAG_SOURCE_H_
 #define SERVICES_UI_WS_DRAG_SOURCE_H_
 
 namespace ui {
 namespace ws {
 
 class DragTargetConnection;
 class ServerWindow;
 struct WindowId;
 
 // An interface implemented by the object that caused a DragController to
 // exist, and which acts as a delegate to it.
 class DragSource {
  public:
   virtual ~DragSource() {}
 
   // 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.
-  virtual void OnDragCompleted(bool success) = 0;
+  // cases where a drag was aborted at any step in the process. |action_taken|
+  // is one of the kDropEffect constants in window_tree_constants.mojom.
+  virtual void OnDragCompleted(bool success, uint32_t action_taken) = 0;
 
   virtual ServerWindow* GetWindowById(const WindowId& id) = 0;
 
   // Returns the client connection that created |window|.
   virtual DragTargetConnection* GetDragTargetForWindow(
       const ServerWindow* window) = 0;
 };
 
 }  // namespace ws
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_DRAG_SOURCE_H_