Move signin status and current user information into AppListModel.

ui/app_list/app_list_model_observer.h

Index: ui/app_list/app_list_model_observer.h
diff --git a/ui/app_list/app_list_model_observer.h b/ui/app_list/app_list_model_observer.h
index c843894c12f8baf15c21dfca2e91735ad865a3ed..07716de735c24e6546cd9641735d9f053e930872 100644
--- a/ui/app_list/app_list_model_observer.h
+++ b/ui/app_list/app_list_model_observer.h
@@ -12,7 +12,13 @@ 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() {}