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

services/ui/public/cpp/window_tree_client.h

 // Copyright 2014 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_PUBLIC_CPP_WINDOW_TREE_CLIENT_H_
 #define SERVICES_UI_PUBLIC_CPP_WINDOW_TREE_CLIENT_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 167 matching lines @@
 
   void PerformDragDrop(
       Window* window,
       int drag_pointer,
       const std::map<std::string, std::vector<uint8_t>>& drag_data,
       int drag_operation,
       const gfx::Point& cursor_location,
       const SkBitmap& bitmap,
       const base::Callback<void(bool, uint32_t)>& callback);
 
+  // Cancels a in progress drag drop. (If no drag is in progress, does
+  // nothing.)
+  void CancelDragDrop(Window* window);
+
   // Performs a window move. |callback| will be asynchronously called with the
   // whether the move loop completed successfully.
   void PerformWindowMove(Window* window,
                          ui::mojom::MoveLoopSource source,
                          const gfx::Point& cursor_location,
                          const base::Callback<void(bool)>& callback);
 
   // Cancels a in progress window move. (If no window is currently being moved,
   // does nothing.)
   void CancelWindowMove(Window* window);
@@ skipping 272 matching lines @@
   std::set<Id> drag_entered_windows_;
 
   base::WeakPtrFactory<WindowTreeClient> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeClient);
 };
 
 }  // namespace ui
 
 #endif  // SERVICES_UI_PUBLIC_CPP_WINDOW_TREE_CLIENT_H_