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

services/ui/ws/event_dispatcher.cc

Index: services/ui/ws/event_dispatcher.cc
diff --git a/services/ui/ws/event_dispatcher.cc b/services/ui/ws/event_dispatcher.cc
index a917e9065c870ea12ee50a7433baf02543cd0d09..fb554ee13af9bfd85138d69f8f0cd8ed26992040 100644
--- a/services/ui/ws/event_dispatcher.cc
+++ b/services/ui/ws/event_dispatcher.cc
@@ -176,6 +176,11 @@ void EventDispatcher::SetDragDropSourceWindow(
       std::move(mime_data), drag_operations);
 }
 
+void EventDispatcher::CancelDragDrop() {
+  if (drag_controller_)
+    drag_controller_->Cancel();
+}
+
 void EventDispatcher::EndDragDrop() {
   drag_controller_.reset();
 }