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

services/ui/ws/window_manager_state.cc

Index: services/ui/ws/window_manager_state.cc
diff --git a/services/ui/ws/window_manager_state.cc b/services/ui/ws/window_manager_state.cc
index 1e5b0ab7c632a721b76bda902e527026ea6ee0c3..93b3d3cde76d15cbce86a6f860f7faea12770a9f 100644
--- a/services/ui/ws/window_manager_state.cc
+++ b/services/ui/ws/window_manager_state.cc
@@ -170,6 +170,10 @@ void WindowManagerState::SetDragDropSourceWindow(
       std::move(drag_data), drag_operation);
 }
 
+void WindowManagerState::CancelDragDrop() {
+  event_dispatcher_.CancelDragDrop();
+}
+
 void WindowManagerState::EndDragDrop() {
   event_dispatcher_.EndDragDrop();
 }