Revert 115515 - [Aura] Implement views-based applist.

ui/base/models/list_model_observer.h

-// Copyright (c) 2011 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_BASE_MODELS_LIST_MODEL_OBSERVER_H_
-#define UI_BASE_MODELS_LIST_MODEL_OBSERVER_H_
-#pragma once
-
-#include "ui/base/ui_export.h"
-
-namespace ui {
-
-class UI_EXPORT ListModelObserver {
- public:
-  // Invoked after items has been added to the model.
-  virtual void ListItemsAdded(int start, int count) = 0;
-
-  // Invoked after items has been removed. |start| is the index before the
-  // removal.
-  virtual void ListItemsRemoved(int start, int count) = 0;
-
-  // Invoked after items has been changed.
-  virtual void ListItemsChanged(int start, int count) = 0;
-
- protected:
-  virtual ~ListModelObserver() {}
-};
-
-}  // namespace ui
-
-#endif  // UI_BASE_MODELS_LIST_MODEL_OBSERVER_H_