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

components/user_manager/user.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_USER_H_
 #define COMPONENTS_USER_MANAGER_USER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 64 matching lines @@
     POLICY = 5,        // Controlled by policy, can't be changed by the user.
     WALLPAPER_TYPE_COUNT = 6
   };
 
   // Returns true if user type has gaia account.
   static bool TypeHasGaiaAccount(UserType user_type);
 
   // Returns the user type.
   virtual UserType GetType() const = 0;
 
-  // The email the user used to log in.
-  const std::string& email() const { return email_; }
-
+/*  // The email the user used to log in.
+  const std::string& GetEmail();
+*/
   // The displayed user name.
   base::string16 display_name() const { return display_name_; }
 
   // If the user has to use SAML to log in.
   bool using_saml() const { return using_saml_; }
 
   // UserInfo
   std::string GetEmail() const override;
   base::string16 GetDisplayName() const override;
   base::string16 GetGivenName() const override;
   const gfx::ImageSkia& GetImage() const override;
-  UserID GetUserID() const override;
+  const UserID& GetUserID() const override;
 
   // Allows managing child status of the user. Used for RegularUser.
   virtual void SetIsChild(bool is_child);
 
   // Returns true if user has gaia account. True for users of types
   // USER_TYPE_REGULAR and USER_TYPE_CHILD.
   virtual bool HasGaiaAccount() const;
 
   // Returns true if user is supervised.
   virtual bool IsSupervised() const;
@@ skipping 62 matching lines @@
   friend class chromeos::UserImageManagerImpl;
   friend class chromeos::UserSessionManager;
 
   // For testing:
   friend class FakeUserManager;
   friend class chromeos::FakeChromeUserManager;
   friend class chromeos::MockUserManager;
   friend class chromeos::UserAddingScreenTest;
 
   // Do not allow anyone else to create new User instances.
-  static User* CreateRegularUser(const UserID& email);
+  static User* CreateRegularUser(const UserID& user_id);
   static User* CreateGuestUser();
-  static User* CreateKioskAppUser(const UserID& kiosk_app_username);
-  static User* CreateSupervisedUser(const UserID& username);
-  static User* CreatePublicAccountUser(const UserID& email);
+  static User* CreateKioskAppUser(const UserID& kiosk_app);
+  static User* CreateSupervisedUser(const UserID& user_id);
+  static User* CreatePublicAccountUser(const UserID& user_id);
 
-  explicit User(const std::string& email);
+  explicit User(const UserID& email);
   ~User() override;
 
   const std::string* GetAccountLocale() const { return account_locale_.get(); }
 
   // Setters are private so only UserManager can call them.
   void SetAccountLocale(const std::string& resolved_account_locale);
 
   void SetImage(const UserImage& user_image, int image_index);
 
   void SetImageURL(const GURL& image_url);
@@ skipping 38 matching lines @@
   void set_can_lock(bool can_lock) { can_lock_ = can_lock; }
 
   void set_is_active(bool is_active) { is_active_ = is_active; }
 
   void set_profile_is_created() { profile_is_created_ = true; }
 
   // True if user has google account (not a guest or managed user).
   bool has_gaia_account() const;
 
  private:
-  std::string email_;
+  UserID user_id_;
   base::string16 display_name_;
   base::string16 given_name_;
   // The displayed user email, defaults to |email_|.
   std::string display_email_;
   bool using_saml_;
   UserImage user_image_;
   OAuthTokenStatus oauth_token_status_;
   bool force_online_signin_;
 
   // This is set to chromeos locale if account data has been downloaded.
@@ skipping 29 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(User);
 };
 
 // List of known users.
 typedef std::vector<User*> UserList;
 
 }  // namespace user_manager
 
 #endif  // COMPONENTS_USER_MANAGER_USER_H_