After uninstall, remove last item in Launcher folder

ui/app_list/app_list_model_observer.h

 // Copyright (c) 2012 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_APP_LIST_APP_LIST_MODEL_OBSERVER_H_
 #define UI_APP_LIST_APP_LIST_MODEL_OBSERVER_H_
 
 #include "ui/app_list/app_list_export.h"
 #include "ui/app_list/app_list_model.h"
 
@@ skipping 12 matching lines @@
 
   // Triggered after |item| has been added to the model.
   virtual void OnAppListItemAdded(AppListItem* item) {}
 
   // Triggered just before an item is deleted from the model.
   virtual void OnAppListItemWillBeDeleted(AppListItem* item) {}
 
   // Triggered just after an item is deleted from the model.
   virtual void OnAppListItemDeleted() {}
 
+  // Triggered after uninstall of 2nd-to-last item in folder, before reparent of
+  // remaining item and folder removal.
+  virtual void OnSingleItemFolderWillBeDeleted(const std::string& folder_id) {}
+
   // Triggered after |item| has moved, changed folders, or changed properties.
   virtual void OnAppListItemUpdated(AppListItem* item) {}
 
   // Triggered when the custom launcher page enabled state is changed.
   virtual void OnCustomLauncherPageEnabledStateChanged(bool enabled) {}
 
   // Triggered when the search engine is changed to and from Google.
   virtual void OnSearchEngineIsGoogleChanged(bool is_google) {}
 
  protected:
   virtual ~AppListModelObserver() {}
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_MODEL_OBSERVER_H_