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

ash/common/system/tray/system_tray_notifier.h

Index: ash/common/system/tray/system_tray_notifier.h
diff --git a/ash/common/system/tray/system_tray_notifier.h b/ash/common/system/tray/system_tray_notifier.h
index 43f19bcc185e896372d7464b7fd97791091a9da4..7b1ccb6af6c3e2e153f77c159784f34e8f0b8fdf 100644
--- a/ash/common/system/tray/system_tray_notifier.h
+++ b/ash/common/system/tray/system_tray_notifier.h
@@ -22,6 +22,8 @@
 class AccessibilityObserver;
 class ClockObserver;
 class IMEObserver;
+struct UpdateInfo;
+class UpdateObserver;
 class UserObserver;
 
 #if defined(OS_CHROMEOS)
@@ -38,10 +40,6 @@
 class TracingObserver;
 class VirtualKeyboardObserver;
 #endif
-
-namespace mojom {
-enum class UpdateSeverity;
-}
 
 // Observer and notification manager for the ash system tray.
 class ASH_EXPORT SystemTrayNotifier {
@@ -68,6 +66,11 @@
   void RemoveIMEObserver(IMEObserver* observer);
   void NotifyRefreshIME();
   void NotifyRefreshIMEMenu(bool is_active);
+
+  // OS updates.
+  void AddUpdateObserver(UpdateObserver* observer);
+  void RemoveUpdateObserver(UpdateObserver* observer);
+  void NotifyUpdateRecommended(const UpdateInfo& info);
 
   // User.
   void AddUserObserver(UserObserver* observer);
@@ -150,6 +153,7 @@
   base::ObserverList<AccessibilityObserver> accessibility_observers_;
   base::ObserverList<ClockObserver> clock_observers_;
   base::ObserverList<IMEObserver> ime_observers_;
+  base::ObserverList<UpdateObserver> update_observers_;
   base::ObserverList<UserObserver> user_observers_;
 
 #if defined(OS_CHROMEOS)