mus drag and drop: Add API to let the client cancel the drag.

services/ui/ws/event_dispatcher.h

 // Copyright 2015 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_EVENT_DISPATCHER_H_
 #define SERVICES_UI_WS_EVENT_DISPATCHER_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 68 matching lines @@
     return capture_window_client_id_;
   }
 
   void SetDragDropSourceWindow(
       DragSource* drag_source,
       ServerWindow* window,
       DragTargetConnection* source_connection,
       int32_t drag_pointer,
       mojo::Map<mojo::String, mojo::Array<uint8_t>> mime_data,
       uint32_t drag_operations);
+  void CancelDragDrop();
   void EndDragDrop();
 
   void OnWillDestroyDragTargetConnection(DragTargetConnection* connection);
 
   // Adds a system modal window. The window remains modal to system until it is
   // destroyed. There can exist multiple system modal windows, in which case the
   // one that is visible and added most recently or shown most recently would be
   // the active one.
   void AddSystemModalWindow(ServerWindow* window);
 
@@ skipping 166 matching lines @@
       AcceleratorMatchPhase::ANY;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(EventDispatcher);
 };
 
 }  // namespace ws
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_EVENT_DISPATCHER_H_