Added Aura notification that relaunch and powerwash is required in case of downgrade.

ash/system/tray_update.h

Index: ash/system/tray_update.h
diff --git a/ash/system/tray_update.h b/ash/system/tray_update.h
index e9fcba9def2098a64e4c0e4b67190bcca7bcf95e..b1d5eb1931f9a20fe3841afd1aad7eeef1cb07da 100644
--- a/ash/system/tray_update.h
+++ b/ash/system/tray_update.h
@@ -5,6 +5,7 @@
 #ifndef ASH_SYSTEM_TRAY_UPDATE_H_
 #define ASH_SYSTEM_TRAY_UPDATE_H_
 
+#include "ash/system/tray/system_tray_delegate.h"
 #include "ash/system/tray/tray_image_item.h"
 #include "ash/system/user/update_observer.h"
 #include "base/memory/scoped_ptr.h"
@@ -33,14 +34,12 @@ class TrayUpdate : public TrayImageItem,
   virtual void DestroyDetailedView() override;
 
   // Overridden from UpdateObserver.
-  virtual void OnUpdateRecommended(UpdateSeverity severity) override;
+  virtual void OnUpdateRecommended(const UpdateInfo& info) override;
 
   // Used to nag the user in case the tray has been hidden too long with an
   // unseen update notification.
   scoped_ptr<tray::UpdateNagger> nagger_;
 
-  UpdateObserver::UpdateSeverity severity_;
-
   DISALLOW_COPY_AND_ASSIGN(TrayUpdate);
 };