We should use UserID object to identify users instead of username.

ash/session/session_state_observer.h

Index: ash/session/session_state_observer.h
diff --git a/ash/session/session_state_observer.h b/ash/session/session_state_observer.h
index 36fad837d611f8b2040a40d13fa76caa886524c0..34613c6a1c1153f1f02a1746c53a92d42fdf1830 100644
--- a/ash/session/session_state_observer.h
+++ b/ash/session/session_state_observer.h
@@ -11,15 +11,19 @@
 #include "ash/session/session_state_delegate.h"
 #include "base/basictypes.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace ash {
 
 class ASH_EXPORT SessionStateObserver {
  public:
   // Called when active user has changed.
-  virtual void ActiveUserChanged(const std::string& user_id) {}
+  virtual void ActiveUserChanged(const user_manager::UserID& user_id) {}
 
   // Called when another user gets added to the existing session.
-  virtual void UserAddedToSession(const std::string& user_id) {}
+  virtual void UserAddedToSession(const user_manager::UserID& user_id) {}
 
   // Called when session state is changed.
   virtual void SessionStateChanged(SessionStateDelegate::SessionState state) {}