ash: Fix system tray avatar not updated regression

ash/common/session/session_state_observer.h

Index: ash/common/session/session_state_observer.h
diff --git a/ash/common/session/session_state_observer.h b/ash/common/session/session_state_observer.h
index 99bdb423e7693a715585965fbeec8b7a52a727b4..83aefd7e74bc175d75949c962079119fcf8bb222 100644
--- a/ash/common/session/session_state_observer.h
+++ b/ash/common/session/session_state_observer.h
@@ -21,6 +21,9 @@ class ASH_EXPORT SessionStateObserver {
   // Called when another user gets added to the existing session.
   virtual void UserAddedToSession(const AccountId& account_id) {}
 
+  // Called when a user session is updated, such as avatar change.
+  virtual void UserSessionUpdated(const AccountId& account_id) {}
+
   // Called when session state is changed.
   virtual void SessionStateChanged(session_manager::SessionState state) {}