mash: Convert ShelfView to wm common types, clean up Shelf access

ash/shelf/shelf_icon_observer.h

-// Copyright 2013 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_SHELF_SHELF_ICON_OBSERVER_H_
-#define ASH_SHELF_SHELF_ICON_OBSERVER_H_
-
-#include "ash/ash_export.h"
-
-namespace ash {
-
-class ASH_EXPORT ShelfIconObserver {
- public:
-  // Invoked when any icon on shelf changes position.
-  virtual void OnShelfIconPositionsChanged() = 0;
-
- protected:
-  virtual ~ShelfIconObserver() {}
-};
-
-}  // namespace ash
-
-#endif  // ASH_SHELF_SHELF_ICON_OBSERVER_H_