Wires up drag/drop for aura-mus

ui/aura/mus/drag_drop_controller_host.h

+// 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.
+
+#ifndef UI_AURA_MUS_DRAG_DROP_CONTROLLER_HOST_H_
+#define UI_AURA_MUS_DRAG_DROP_CONTROLLER_HOST_H_
+
+#include <stdint.h>
+
+#include "ui/aura/aura_export.h"
+
+namespace aura {
+
+class WindowMus;
+
+// Used by DragDropControllerMus to create a change id when a drag starts that
+// is sent to ui::mojom::WindowTree.
+class AURA_EXPORT DragDropControllerHost {
+ public:
+  virtual uint32_t CreateChangeIdForDrag(WindowMus* window) = 0;
+
+ protected:
+  virtual ~DragDropControllerHost() {}
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_MUS_DRAG_DROP_CONTROLLER_HOST_H_