Adding "UserAddedToSession" notifier to the SessionStateObserver

ash/session_state_observer.h

Index: ash/session_state_observer.h
diff --git a/ash/session_state_observer.h b/ash/session_state_observer.h
index f4fad9dbf913f9e37e1bb963b5fe32669285afe5..1de31cd1db72b0e6ef1ff54ec7447804ff921a8e 100644
--- a/ash/session_state_observer.h
+++ b/ash/session_state_observer.h
@@ -16,6 +16,9 @@ class ASH_EXPORT SessionStateObserver {
   // Called when active user has changed.
   virtual void ActiveUserChanged(const std::string& user_id) {}
 
+  // Called when another user gets added to the existing session.
+  virtual void UserAddedToSession(const std::string& user_id) {}
+
  protected:
   virtual ~SessionStateObserver() {}
 };