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

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

Index: chrome/browser/chromeos/login/users/fake_supervised_user_manager.h
diff --git a/chrome/browser/chromeos/login/users/fake_supervised_user_manager.h b/chrome/browser/chromeos/login/users/fake_supervised_user_manager.h
index d2a9a4389b2f85cf024bcbd29d13051fd1929fd9..fa738f9cf410abae5a9021c2ab769b2a2aed75ed 100644
--- a/chrome/browser/chromeos/login/users/fake_supervised_user_manager.h
+++ b/chrome/browser/chromeos/login/users/fake_supervised_user_manager.h
@@ -9,6 +9,10 @@
 #include "base/compiler_specific.h"
 #include "chrome/browser/chromeos/login/users/supervised_user_manager.h"
 
+namespace user_manager {
+class UserID;
+} // namespace user_manager
+
 namespace chromeos {
 
 // Fake supervised user manager with a barebones implementation.
@@ -17,10 +21,10 @@ class FakeSupervisedUserManager : public SupervisedUserManager {
   FakeSupervisedUserManager();
   ~FakeSupervisedUserManager() 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;
@@ -28,19 +32,19 @@ class FakeSupervisedUserManager : public SupervisedUserManager {
       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;
+  const 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;