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

ash/system/tray/system_tray_delegate.h

Index: ash/system/tray/system_tray_delegate.h
diff --git a/ash/system/tray/system_tray_delegate.h b/ash/system/tray/system_tray_delegate.h
index 857ee09359497ebe0b25abc5558b9eaf02b4d84b..bb5709019e088b7c9cf3e2b194f5989ce708092f 100644
--- a/ash/system/tray/system_tray_delegate.h
+++ b/ash/system/tray/system_tray_delegate.h
@@ -23,6 +23,10 @@ class TimeDelta;
 class TimeTicks;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace ash {
 
 class CustodianInfoTrayObserver;
@@ -317,7 +321,7 @@ class ASH_EXPORT SystemTrayDelegate {
 
   // Returns accounts delegate for given user. May return nullptr.
   virtual tray::UserAccountsDelegate* GetUserAccountsDelegate(
-      const std::string& user_id);
+      const user_manager::UserID& user_id);
 
   // Adding observers that are notified when supervised info is being changed.
   virtual void AddCustodianInfoTrayObserver(