ash: Use system tray mojo interface to show system update tray icon

ash/common/system/tray/system_tray_controller.h

Index: ash/common/system/tray/system_tray_controller.h
diff --git a/ash/common/system/tray/system_tray_controller.h b/ash/common/system/tray/system_tray_controller.h
index eda45914a354e184d566a628a68cbdc34c0382ae..e1771cfd2473d853332489bbef5ca589913724a6 100644
--- a/ash/common/system/tray/system_tray_controller.h
+++ b/ash/common/system/tray/system_tray_controller.h
@@ -56,11 +56,13 @@ class ASH_EXPORT SystemTrayController
   // Binds the mojom::SystemTray interface to this object.
   void BindRequest(mojom::SystemTrayRequest request);
 
- private:
-  // mojom::SystemTray:
+  // mojom::SystemTray overrides. Public for testing.
   void SetClient(mojom::SystemTrayClientPtr client) override;
   void SetUse24HourClock(bool use_24_hour) override;
+  void ShowUpdateIcon(mojom::UpdateSeverity severity,
+                      bool factory_reset_required) override;
 
+ private:
   // Client interface in chrome browser. Only bound on Chrome OS.
   mojom::SystemTrayClientPtr system_tray_client_;