Move signin status and current user information into AppListModel.

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"
 
 namespace app_list {
 
 class APP_LIST_EXPORT AppListModelObserver {
  public:
   // Invoked when AppListModel's status has changed.
-  virtual void OnAppListModelStatusChanged() = 0;
+  virtual void OnAppListModelStatusChanged() {}
+
+  // Invoked when AppListModel's current user details have changed.
+  virtual void OnAppListModelCurrentUserChanged() {}
+
+  // Invoked when AppListModel's current user's signin status has changed.
+  virtual void OnAppListModelSigninStatusChanged() {}
 
  protected:
   virtual ~AppListModelObserver() {}
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_MODEL_OBSERVER_H_