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

chrome/browser/chromeos/login/supervised/supervised_user_authenticator.h

Index: chrome/browser/chromeos/login/supervised/supervised_user_authenticator.h
diff --git a/chrome/browser/chromeos/login/supervised/supervised_user_authenticator.h b/chrome/browser/chromeos/login/supervised/supervised_user_authenticator.h
index 4402946764d3c985719265f62097c8141350d030..da3dd2057e12352683094d692a240a8c749b55ab 100644
--- a/chrome/browser/chromeos/login/supervised/supervised_user_authenticator.h
+++ b/chrome/browser/chromeos/login/supervised/supervised_user_authenticator.h
@@ -11,6 +11,7 @@
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
+#include "components/user_manager/user_id.h"
 #include "third_party/cros_system_api/dbus/service_constants.h"
 
 namespace chromeos {
@@ -33,7 +34,7 @@ class SupervisedUserAuthenticator
 
   class AuthAttempt {
    public:
-    AuthAttempt(const std::string& username,
+    AuthAttempt(const user_manager::UserID& user_id,
                 const std::string& password,
                 bool add_key_attempt);
     ~AuthAttempt();
@@ -55,7 +56,7 @@ class SupervisedUserAuthenticator
     std::string hash();
     cryptohome::MountError cryptohome_code();
 
-    const std::string username;
+    const user_manager::UserID user_id;
     const std::string password;
     const bool add_key;
 
@@ -82,13 +83,13 @@ class SupervisedUserAuthenticator
 
   explicit SupervisedUserAuthenticator(AuthStatusConsumer* consumer);
 
-  void AuthenticateToMount(const std::string& username,
+  void AuthenticateToMount(const user_manager::UserID& user_id,
                            const std::string& password);
 
-  void AuthenticateToCreate(const std::string& username,
+  void AuthenticateToCreate(const user_manager::UserID& user_id,
                             const std::string& password);
 
-  void AddMasterKey(const std::string& username,
+  void AddMasterKey(const user_manager::UserID& user_id,
                     const std::string& password,
                     const std::string& master_key);
   void Resolve();