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

chromeos/login/auth/user_context.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 CHROMEOS_LOGIN_AUTH_USER_CONTEXT_H_
 #define CHROMEOS_LOGIN_AUTH_USER_CONTEXT_H_
 
 #include <string>
 
 #include "chromeos/chromeos_export.h"
 #include "chromeos/login/auth/key.h"
+#include "components/user_manager/user_id.h"
 #include "components/user_manager/user_type.h"
 
 namespace chromeos {
 
 // Information that is passed around while authentication is in progress. The
 // credentials may consist of a |user_id_|, |key_| pair or a GAIA |auth_code_|.
 // The |user_id_hash_| is used to locate the user's home directory
 // mount point for the user. It is set when the mount has been completed.
 class CHROMEOS_EXPORT UserContext {
  public:
   // The authentication flow used during sign-in.
   enum AuthFlow {
     // Online authentication against GAIA. GAIA did not redirect to a SAML IdP.
     AUTH_FLOW_GAIA_WITHOUT_SAML,
     // Online authentication against GAIA. GAIA redirected to a SAML IdP.
     AUTH_FLOW_GAIA_WITH_SAML,
     // Offline authentication against a cached key.
     AUTH_FLOW_OFFLINE,
     // Offline authentication using and Easy unlock device (e.g. a phone).
     AUTH_FLOW_EASY_UNLOCK,
     // Easy bootstrap flow.
     AUTH_FLOW_EASY_BOOTSTRAP
   };
 
   UserContext();
   UserContext(const UserContext& other);
-  explicit UserContext(const std::string& user_id);
-  UserContext(user_manager::UserType user_type, const std::string& user_id);
+  explicit UserContext(const user_manager::UserID& user_id);
+  UserContext(user_manager::UserType user_type, const user_manager::UserID& user_id);
   ~UserContext();
 
   bool operator==(const UserContext& context) const;
   bool operator!=(const UserContext& context) const;
 
-  const std::string& GetUserID() const;
-  const std::string& GetGaiaID() const;
+  const user_manager::UserID& GetUserID() const;
   const Key* GetKey() const;
   Key* GetKey();
   const std::string& GetAuthCode() const;
   const std::string& GetRefreshToken() const;
   const std::string& GetAccessToken() const;
   const std::string& GetUserIDHash() const;
   bool IsUsingOAuth() const;
   AuthFlow GetAuthFlow() const;
   user_manager::UserType GetUserType() const;
   const std::string& GetPublicSessionLocale() const;
   const std::string& GetPublicSessionInputMethod() const;
   const std::string& GetDeviceId() const;
 
   bool HasCredentials() const;
 
-  void SetUserID(const std::string& user_id);
-  void SetGaiaID(const std::string& gaia_id);
+  void SetUserID(const user_manager::UserID& user_id);
   void SetKey(const Key& key);
   void SetAuthCode(const std::string& auth_code);
   void SetRefreshToken(const std::string& refresh_token);
   void SetAccessToken(const std::string& access_token);
   void SetUserIDHash(const std::string& user_id_hash);
   void SetIsUsingOAuth(bool is_using_oauth);
   void SetAuthFlow(AuthFlow auth_flow);
   void SetUserType(user_manager::UserType user_type);
   void SetPublicSessionLocale(const std::string& locale);
   void SetPublicSessionInputMethod(const std::string& input_method);
   void SetDeviceId(const std::string& device_id);
 
   void ClearSecrets();
 
  private:
-  std::string user_id_;
-  std::string gaia_id_;
+  user_manager::UserID user_id_;
   Key key_;
   std::string auth_code_;
   std::string refresh_token_;
   std::string access_token_;  // OAuthLogin scoped access token.
   std::string user_id_hash_;
   bool is_using_oauth_;
   AuthFlow auth_flow_;
   user_manager::UserType user_type_;
   std::string public_session_locale_;
   std::string public_session_input_method_;
   std::string device_id_;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_LOGIN_AUTH_USER_CONTEXT_H_