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

chrome/browser/chromeos/login/users/supervised_user_manager_impl.h

Index: chrome/browser/chromeos/login/users/supervised_user_manager_impl.h
diff --git a/chrome/browser/chromeos/login/users/supervised_user_manager_impl.h b/chrome/browser/chromeos/login/users/supervised_user_manager_impl.h
index 06ecc4a12dbd063af1ccdc8a07a1cc452b95dc69..03bb78400a441bd6149c240b21e178c7b0d54453 100644
--- a/chrome/browser/chromeos/login/users/supervised_user_manager_impl.h
+++ b/chrome/browser/chromeos/login/users/supervised_user_manager_impl.h
@@ -24,30 +24,30 @@ class SupervisedUserManagerImpl
  public:
   ~SupervisedUserManagerImpl() override;
 
-  bool HasSupervisedUsers(const std::string& manager_id) const override;
+  bool HasSupervisedUsers(const user_manager::UserID& manager_id) const override;
   const user_manager::User* CreateUserRecord(
-      const std::string& manager_id,
-      const std::string& local_user_id,
+      const user_manager::UserID& manager_id,
+      const user_manager::UserID& local_user_id,
       const std::string& sync_user_id,
       const base::string16& display_name) override;
-  std::string GenerateUserId() override;
+  user_manager::UserID GenerateUserId() override;
   const user_manager::User* FindByDisplayName(
       const base::string16& display_name) const override;
   const user_manager::User* FindBySyncId(
       const std::string& sync_id) const override;
-  std::string GetUserSyncId(const std::string& user_id) const override;
+  std::string GetUserSyncId(const user_manager::UserID& user_id) const override;
   base::string16 GetManagerDisplayName(
-      const std::string& user_id) const override;
-  std::string GetManagerUserId(const std::string& user_id) const override;
-  std::string GetManagerDisplayEmail(const std::string& user_id) const override;
+      const user_manager::UserID& user_id) const override;
+  user_manager::UserID GetManagerUserId(const user_manager::UserID& user_id) const override;
+  std::string GetManagerDisplayEmail(const user_manager::UserID& user_id) const override;
   void StartCreationTransaction(const base::string16& display_name) override;
-  void SetCreationTransactionUserId(const std::string& user_id) override;
+  void SetCreationTransactionUserId(const user_manager::UserID& user_id) override;
   void CommitCreationTransaction() override;
   SupervisedUserAuthentication* GetAuthentication() override;
-  void GetPasswordInformation(const std::string& user_id,
+  void GetPasswordInformation(const user_manager::UserID& user_id,
                               base::DictionaryValue* result) override;
   void SetPasswordInformation(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       const base::DictionaryValue* password_info) override;
   void LoadSupervisedUserToken(Profile* profile,
                                const LoadTokenCallback& callback) override;
@@ -67,39 +67,39 @@ class SupervisedUserManagerImpl
   // Attempts to clean up data that could be left from failed user creation.
   void RollbackUserCreationTransaction();
 
-  void RemoveNonCryptohomeData(const std::string& user_id);
+  void RemoveNonCryptohomeData(const user_manager::UserID& user_id);
 
-  bool CheckForFirstRun(const std::string& user_id);
+  bool CheckForFirstRun(const user_manager::UserID& user_id);
 
   // Update name if this user is manager of some managed users.
-  void UpdateManagerName(const std::string& manager_id,
+  void UpdateManagerName(const user_manager::UserID& manager_id,
                          const base::string16& new_display_name);
 
-  bool GetUserStringValue(const std::string& user_id,
+  bool GetUserStringValue(const user_manager::UserID& user_id,
                           const char* key,
                           std::string* out_value) const;
 
-  void SetUserStringValue(const std::string& user_id,
+  void SetUserStringValue(const user_manager::UserID& user_id,
                           const char* key,
                           const std::string& value);
 
-  bool GetUserIntegerValue(const std::string& user_id,
+  bool GetUserIntegerValue(const user_manager::UserID& user_id,
                            const char* key,
                            int* out_value) const;
 
-  void SetUserIntegerValue(const std::string& user_id,
+  void SetUserIntegerValue(const user_manager::UserID& user_id,
                            const char* key,
                            const int value);
 
-  bool GetUserBooleanValue(const std::string& user_id,
+  bool GetUserBooleanValue(const user_manager::UserID& user_id,
                            const char* key,
                            bool* out_value) const;
 
-  void SetUserBooleanValue(const std::string& user_id,
+  void SetUserBooleanValue(const user_manager::UserID& user_id,
                            const char* key,
                            const bool value);
 
-  void CleanPref(const std::string& user_id,
+  void CleanPref(const user_manager::UserID& user_id,
                  const char* key);
 
   ChromeUserManagerImpl* owner_;