aura-mus: wire up drag and drop.

services/ui/ws/drag_controller.h

Index: services/ui/ws/drag_controller.h
diff --git a/services/ui/ws/drag_controller.h b/services/ui/ws/drag_controller.h
index cb402829e82c19586b25638a94f06ed4c3e016e5..edbc11a703532f1ab877acb864746b496cca6c36 100644
--- a/services/ui/ws/drag_controller.h
+++ b/services/ui/ws/drag_controller.h
@@ -111,6 +111,8 @@ class DragController : public ServerWindowObserver {
   // ServerWindowObserver:
   void OnWindowDestroying(ServerWindow* window) override;
 
+  static std::string ToString(OperationType type);
+
   // Our owner.
   DragSource* source_;