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

components/user_manager/fake_user_manager.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_USER_MANAGER_FAKE_USER_MANAGER_H_
 #define COMPONENTS_USER_MANAGER_FAKE_USER_MANAGER_H_
 
 #include <map>
 #include <string>
 
@@ skipping 26 matching lines @@
                     const std::string& username_hash,
                     bool browser_restart) override;
 
   const user_manager::User* GetActiveUser() const override;
   user_manager::User* GetActiveUser() override;
   void SwitchActiveUser(const std::string& email) override;
   void SaveUserDisplayName(const std::string& username,
                            const base::string16& display_name) override;
 
   // Not implemented.
-  void UpdateUserAccountData(const std::string& user_id,
+  void UpdateUserAccountData(const UserID& user_id,
                              const UserAccountData& account_data) override {}
   void Shutdown() override {}
   const user_manager::UserList& GetLRULoggedInUsers() const override;
   user_manager::UserList GetUnlockUsers() const override;
   const std::string& GetOwnerEmail() const override;
   void SessionStarted() override {}
   void RemoveUser(const std::string& email,
                   user_manager::RemoveUserDelegate* delegate) override {}
   void RemoveUserFromList(const std::string& email) override;
   bool IsKnownUser(const std::string& email) const override;
   const user_manager::User* FindUser(const std::string& email) const override;
   user_manager::User* FindUserAndModify(const std::string& email) override;
   const user_manager::User* GetLoggedInUser() const override;
   user_manager::User* GetLoggedInUser() override;
   const user_manager::User* GetPrimaryUser() const override;
   void SaveUserOAuthStatus(
       const std::string& username,
       user_manager::User::OAuthTokenStatus oauth_token_status) override {}
-  void SaveForceOnlineSignin(const std::string& user_id,
+  void SaveForceOnlineSignin(const UserID& user_id,
                              bool force_online_signin) override {}
   base::string16 GetUserDisplayName(const std::string& username) const override;
   void SaveUserDisplayEmail(const std::string& username,
                             const std::string& display_email) override {}
   std::string GetUserDisplayEmail(const std::string& username) const override;
   bool IsCurrentUserOwner() const override;
   bool IsCurrentUserNew() const override;
   bool IsCurrentUserNonCryptohomeDataEphemeral() const override;
   bool CanCurrentUserLock() const override;
   bool IsUserLoggedIn() const override;
@@ skipping 11 matching lines @@
   void AddSessionStateObserver(UserSessionStateObserver* obs) override {}
   void RemoveSessionStateObserver(UserSessionStateObserver* obs) override {}
   void NotifyLocalStateChanged() override {}
   bool AreSupervisedUsersAllowed() const override;
 
   // UserManagerBase overrides:
   bool AreEphemeralUsersEnabled() const override;
   const std::string& GetApplicationLocale() const override;
   PrefService* GetLocalState() const override;
   void HandleUserOAuthTokenStatusChange(
-      const std::string& user_id,
+      const UserID& user_id,
       user_manager::User::OAuthTokenStatus status) const override {}
   bool IsEnterpriseManaged() const override;
   void LoadPublicAccounts(std::set<std::string>* public_sessions_set) override {
   }
   void PerformPreUserListLoadingActions() override {}
   void PerformPostUserListLoadingActions() override {}
   void PerformPostUserLoggedInActions(bool browser_restart) override {}
-  bool IsDemoApp(const std::string& user_id) const override;
-  bool IsKioskApp(const std::string& user_id) const override;
+  bool IsDemoApp(const UserID& user_id) const override;
+  bool IsKioskApp(const UserID& user_id) const override;
   bool IsPublicAccountMarkedForRemoval(
-      const std::string& user_id) const override;
+      const UserID& user_id) const override;
   void DemoAccountLoggedIn() override {}
   void KioskAppLoggedIn(const std::string& app_id) override {}
   void PublicAccountUserLoggedIn(user_manager::User* user) override {}
-  void SupervisedUserLoggedIn(const std::string& user_id) override {}
+  void SupervisedUserLoggedIn(const UserID& user_id) override {}
 
  protected:
   user_manager::User* primary_user_;
 
   // If set this is the active user. If empty, the first created user is the
   // active user.
-  std::string active_user_id_;
+  UserID active_user_id_;
 
  private:
   // We use this internal function for const-correctness.
   user_manager::User* GetActiveUserInternal() const;
 
   // stub, always empty string.
   std::string owner_email_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeUserManager);
 };
 
 }  // namespace user_manager
 
 #endif  // COMPONENTS_USER_MANAGER_FAKE_USER_MANAGER_H_