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

services/ui/ws/window_manager_state.cc

 // Copyright 2016 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/ws/window_manager_state.h"
 
 #include <utility>
 
 #include "base/memory/weak_ptr.h"
 #include "services/shell/public/interfaces/connector.mojom.h"
@@ skipping 152 matching lines @@
     ServerWindow* window,
     DragTargetConnection* source_connection,
     int32_t drag_pointer,
     mojo::Map<mojo::String, mojo::Array<uint8_t>> drag_data,
     uint32_t drag_operation) {
   event_dispatcher_.SetDragDropSourceWindow(
       drag_source, window, source_connection, drag_pointer,
       std::move(drag_data), drag_operation);
 }
 
+void WindowManagerState::CancelDragDrop() {
+  event_dispatcher_.CancelDragDrop();
+}
+
 void WindowManagerState::EndDragDrop() {
   event_dispatcher_.EndDragDrop();
 }
 
 void WindowManagerState::AddSystemModalWindow(ServerWindow* window) {
   DCHECK(!window->transient_parent());
   event_dispatcher_.AddSystemModalWindow(window);
 }
 
 const UserId& WindowManagerState::user_id() const {
@@ skipping 432 matching lines @@
       window->RemoveObserver(this);
       orphaned_window_manager_display_roots_.erase(iter);
       return;
     }
   }
   NOTREACHED();
 }
 
 }  // namespace ws
 }  // namespace ui