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

services/ui/public/cpp/window.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_H_
 #define SERVICES_UI_PUBLIC_CPP_WINDOW_H_
 
 #include <stdint.h>
 #include <vector>
 
@@ skipping 225 matching lines @@
   // is one of the ui::mojom::kDropEffect constants in
   // window_tree_constants.mojom.
   void PerformDragDrop(
       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 the in progress drag started with PerformDragDrop().
+  void CancelDragDrop();
+
   // Tells the window manager to take control of moving the window. Returns
   // true if the move wasn't canceled.
   void PerformWindowMove(mojom::MoveLoopSource source,
                          const gfx::Point& cursor_location,
                          const base::Callback<void(bool)>& callback);
 
   // Tells the window manager to abort any current move initiated by
   // PerformWindowMove().
   void CancelWindowMove();
 
@@ skipping 130 matching lines @@
   };
 
   std::map<const void*, Value> prop_map_;
 
   DISALLOW_COPY_AND_ASSIGN(Window);
 };
 
 }  // namespace ui
 
 #endif  // SERVICES_UI_PUBLIC_CPP_WINDOW_H_