Introduce a type of View background that stays in sync with its host

ash/system/user/user_view.h

Index: ash/system/user/user_view.h
diff --git a/ash/system/user/user_view.h b/ash/system/user/user_view.h
index 535a4b5077ae878514e32440428f1868793b1eb3..de498772330bb620f4501c34de8927b22b41b5fc 100644
--- a/ash/system/user/user_view.h
+++ b/ash/system/user/user_view.h
@@ -62,10 +62,10 @@ class UserView : public views::View,
 
   // Create the menu option to add another user. If |disabled| is set the user
   // cannot actively click on the item.
-  void ToggleAddUserMenuOption();
+  void ToggleUserDropdownWidget();
 
   // Removes the add user menu option.
-  void RemoveAddUserMenuOption();
+  void HideUserDropdownWidget();
 
   // If |user_card_view_| is clickable, this is a ButtonFromView that wraps it.
   // If |user_card_view_| is not clickable, this will be equal to
@@ -79,7 +79,7 @@ class UserView : public views::View,
   SystemTrayItem* owner_;
 
   views::View* logout_button_ = nullptr;
-  std::unique_ptr<views::Widget> add_menu_option_;
+  std::unique_ptr<views::Widget> user_dropdown_widget_;
 
   // False when the add user panel is visible but not activatable.
   bool add_user_enabled_ = true;