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

ash/session/session_state_delegate.h

Index: ash/session/session_state_delegate.h
diff --git a/ash/session/session_state_delegate.h b/ash/session/session_state_delegate.h
index 5e73e58310c225049f33b2c449e0a91d053ea1ef..c7e1de432c2e0b7a5e7c925cb9c1c2f310924877 100644
--- a/ash/session/session_state_delegate.h
+++ b/ash/session/session_state_delegate.h
@@ -25,6 +25,7 @@ class ImageSkia;
 
 namespace user_manager {
 class UserInfo;
+class UserID;
 }  // namespace user_manager
 
 namespace ash {
@@ -144,7 +145,7 @@ class ASH_EXPORT SessionStateDelegate {
 
   // Switches to another active user with |user_id|
   // (if that user has already signed in).
-  virtual void SwitchActiveUser(const std::string& user_id) = 0;
+  virtual void SwitchActiveUser(const user_manager::UserID& user_id) = 0;
 
   // Switches the active user to the next or previous user, with the same
   // ordering as GetLoggedInUsers.