ash: Update system update notification behaviour.

ash/system/user/update_observer.h

Index: ash/system/user/update_observer.h
diff --git a/ash/system/user/update_observer.h b/ash/system/user/update_observer.h
index 066a7bc780ebe18346fa1182d82dabcd9677eb6c..e509f62d3cb31461a1ba671d7478cbf4a79f9f5e 100644
--- a/ash/system/user/update_observer.h
+++ b/ash/system/user/update_observer.h
@@ -11,9 +11,16 @@ namespace ash {
 
 class ASH_EXPORT UpdateObserver {
  public:
+  enum UpdateSeverity {
+    UPDATE_NORMAL,
+    UPDATE_LOW_GREEN,
+    UPDATE_HIGH_ORANGE,
+    UPDATE_SEVERE_RED,
+  };
+
   virtual ~UpdateObserver() {}
 
-  virtual void OnUpdateRecommended() = 0;
+  virtual void OnUpdateRecommended(UpdateSeverity severity) = 0;
 };
 
 }  // namespace ash