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

services/ui/ws/window_tree.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_WS_WINDOW_TREE_H_
 #define SERVICES_UI_WS_WINDOW_TREE_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 430 matching lines @@
   void GetWindowManagerClient(
       mojo::AssociatedInterfaceRequest<mojom::WindowManagerClient> internal)
       override;
   void GetCursorLocationMemory(const GetCursorLocationMemoryCallback& callback)
       override;
   void PerformDragDrop(uint32_t change_id,
                        Id source_window_id,
                        int32_t drag_pointer,
                        mojo::Map<mojo::String, mojo::Array<uint8_t>> drag_data,
                        uint32_t drag_operation) override;
+  void CancelDragDrop(Id window_id) override;
   void PerformWindowMove(uint32_t change_id,
                          Id window_id,
                          ui::mojom::MoveLoopSource source,
                          const gfx::Point& cursor) override;
   void CancelWindowMove(Id window_id) override;
 
   // mojom::WindowManagerClient:
   void AddAccelerator(uint32_t id,
                       mojom::EventMatcherPtr event_matcher,
                       const AddAcceleratorCallback& callback) override;
@@ skipping 106 matching lines @@
       waiting_for_top_level_window_info_;
   bool embedder_intercepts_events_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTree);
 };
 
 }  // namespace ws
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_WINDOW_TREE_H_