mash: Port ash_sysui ShelfDelegateMus impl to mojo:ash.

ash/mus/shelf_layout_manager_delegate.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 ASH_MUS_SHELF_LAYOUT_MANAGER_DELEGATE_H_
-#define ASH_MUS_SHELF_LAYOUT_MANAGER_DELEGATE_H_
-
-namespace ash {
-namespace mus {
-
-class ShelfLayoutManagerDelegate {
- public:
-  // Called when the window associated with the shelf is available. That is,
-  // ShelfLayoutManager::GetShelfWindow() returns non-null.
-  virtual void OnShelfWindowAvailable() = 0;
-
- protected:
-  virtual ~ShelfLayoutManagerDelegate() {}
-};
-
-}  // namespace mus
-}  // namespace ash
-
-#endif  // ASH_MUS_SHELF_LAYOUT_MANAGER_DELEGATE_H_