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

components/user_manager/fake_user_manager.h

Index: components/user_manager/fake_user_manager.h
diff --git a/components/user_manager/fake_user_manager.h b/components/user_manager/fake_user_manager.h
index c71df24e80d7f8dad99553027cae51b1d762cf05..0c414e8b6cf69b222680c3c64200476469a33e52 100644
--- a/components/user_manager/fake_user_manager.h
+++ b/components/user_manager/fake_user_manager.h
@@ -44,7 +44,7 @@ class USER_MANAGER_EXPORT FakeUserManager : public UserManagerBase {
                            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;
@@ -63,7 +63,7 @@ class USER_MANAGER_EXPORT FakeUserManager : public UserManagerBase {
   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,
@@ -95,7 +95,7 @@ class USER_MANAGER_EXPORT FakeUserManager : public UserManagerBase {
   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 {
@@ -103,21 +103,21 @@ class USER_MANAGER_EXPORT FakeUserManager : public UserManagerBase {
   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.