Use Ash's ShelfWindowWatcher for app panel windows.

ash/common/shelf/shelf_model.cc

 // 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.
 
 #include "ash/common/shelf/shelf_model.h"
 
 #include <algorithm>
 
 #include "ash/common/shelf/shelf_item_delegate.h"
 #include "ash/common/shelf/shelf_model_observer.h"
@@ skipping 53 matching lines @@
   index = ValidateInsertionIndex(item.type, index);
   items_.insert(items_.begin() + index, item);
   items_[index].id = next_id_++;
   for (auto& observer : observers_)
     observer.ShelfItemAdded(index);
   return index;
 }
 
 void ShelfModel::RemoveItemAt(int index) {
   DCHECK(index >= 0 && index < item_count());
-  // The app list and browser shortcut can't be removed.
-  DCHECK(items_[index].type != TYPE_APP_LIST &&
-         items_[index].type != TYPE_BROWSER_SHORTCUT);
   ShelfID id = items_[index].id;
   items_.erase(items_.begin() + index);
   RemoveShelfItemDelegate(id);
   // TODO(jamescook): Fold this into ShelfItemRemoved in existing observers.
   for (auto& observer : observers_)
     observer.OnSetShelfItemDelegate(id, nullptr);
   for (auto& observer : observers_)
     observer.ShelfItemRemoved(index, id);
 }
 
@@ skipping 121 matching lines @@
 
   return index;
 }
 
 void ShelfModel::RemoveShelfItemDelegate(ShelfID id) {
   if (id_to_item_delegate_map_.find(id) != id_to_item_delegate_map_.end())
     id_to_item_delegate_map_.erase(id);
 }
 
 }  // namespace ash