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

components/user_manager/user.h

Index: components/user_manager/user.h
diff --git a/components/user_manager/user.h b/components/user_manager/user.h
index a0b2c2742a2f2d806843ea92969d6f561a748d91..fe774b43e5c71b9c54ec95d5aa1164333df80c44 100644
--- a/components/user_manager/user.h
+++ b/components/user_manager/user.h
@@ -82,9 +82,9 @@ class USER_MANAGER_EXPORT User : public UserInfo {
   // 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_; }
 
@@ -96,7 +96,7 @@ class USER_MANAGER_EXPORT User : public UserInfo {
   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);
@@ -179,13 +179,13 @@ class USER_MANAGER_EXPORT User : public UserInfo {
   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(); }
@@ -244,7 +244,7 @@ class USER_MANAGER_EXPORT User : public UserInfo {
   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_|.