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

services/ui/public/cpp/window.cc

 // 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.
 
 #include "services/ui/public/cpp/window.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <set>
@@ skipping 474 matching lines @@
     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) {
   client_->PerformDragDrop(this, drag_pointer, drag_data, drag_operation,
                            cursor_location, bitmap, callback);
 }
 
+void Window::CancelDragDrop() {
+  client_->CancelDragDrop(this);
+}
+
 void Window::PerformWindowMove(mojom::MoveLoopSource source,
                                const gfx::Point& cursor_location,
                                const base::Callback<void(bool)>& callback) {
   client_->PerformWindowMove(this, source, cursor_location, callback);
 }
 
 void Window::CancelWindowMove() {
   client_->CancelWindowMove(this);
 }
 
@@ skipping 416 matching lines @@
     notifier->NotifyWindowReordered();
 
   return true;
 }
 
 // static
 Window** Window::GetStackingTarget(Window* window) {
   return &window->stacking_target_;
 }
 }  // namespace ui