cros: Use SessionController to enable system tray settings / notifications tray

ash/public/interfaces/session_controller.mojom

Index: ash/public/interfaces/session_controller.mojom
diff --git a/ash/public/interfaces/session_controller.mojom b/ash/public/interfaces/session_controller.mojom
index 6a2e43834d664657e5f658049c9f301516819130..65ead11571def1837c88a005830b30322f735258 100644
--- a/ash/public/interfaces/session_controller.mojom
+++ b/ash/public/interfaces/session_controller.mojom
@@ -84,6 +84,14 @@ struct UserSession {
   string display_name;
   string display_email;
   gfx.mojom.ImageSkia avatar;
+
+  // Whether the settings icon should be enabled in the system tray menu.
+  // Usually true after login, but can be false for specialized user sessions
+  // (e.g. adding supervised users).
+  bool should_enable_settings;
+
+  // Similar to |should_show_settings| but for the notification tray.
+  bool should_show_notification_tray;
 };
 
 // Matches ash::AddUserSessionPolicy.