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

components/user_manager/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_USER_MANAGER_H_
 #define COMPONENTS_USER_MANAGER_USER_MANAGER_H_
 
 #include <string>
 
 #include "components/user_manager/user.h"
@@ skipping 120 matching lines @@
   virtual const UserList& GetLRULoggedInUsers() const = 0;
 
   // Returns a list of users who can unlock the device.
   // This list is based on policy and whether user is able to do unlock.
   // Policy:
   // * If user has primary-only policy then it is the only user in unlock users.
   // * Otherwise all users with unrestricted policy are added to this list.
   // All users that are unable to perform unlock are excluded from this list.
   virtual UserList GetUnlockUsers() const = 0;
 
-  // Returns the email of the owner user. Returns an empty string if there is
+  // Returns the id of the owner user. Returns an empty id if there is
   // no owner for the device.
-  virtual const std::string& GetOwnerEmail() const = 0;
+  virtual const UserID& GetOwnerID() const = 0;
 
   // Indicates that a user with the given |user_id| has just logged in. The
   // persistent list is updated accordingly if the user is not ephemeral.
   // |browser_restart| is true when reloading Chrome after crash to distinguish
   // from normal sign in flow.
   // |username_hash| is used to identify homedir mount point.
-  virtual void UserLoggedIn(const std::string& user_id,
+  virtual void UserLoggedIn(const UserID& user_id,
                             const std::string& username_hash,
                             bool browser_restart) = 0;
 
   // Switches to active user identified by |user_id|. User has to be logged in.
-  virtual void SwitchActiveUser(const std::string& user_id) = 0;
+  virtual void SwitchActiveUser(const UserID& user_id) = 0;
 
   // Switches to the last active user (called after crash happens and session
   // restore has completed).
   virtual void SwitchToLastActiveUser() = 0;
 
   // Called when browser session is started i.e. after
   // browser_creator.LaunchBrowser(...) was called after user sign in.
   // When user is at the image screen IsUserLoggedIn() will return true
   // but IsSessionStarted() will return false. During the kiosk splash screen,
   // we perform additional initialization after the user is logged in but
   // before the session has been started.
   // Fires NOTIFICATION_SESSION_STARTED.
   virtual void SessionStarted() = 0;
 
   // Removes the user from the device. Note, it will verify that the given user
   // isn't the owner, so calling this method for the owner will take no effect.
   // Note, |delegate| can be NULL.
-  virtual void RemoveUser(const std::string& user_id,
+  virtual void RemoveUser(const UserID& user_id,
                           RemoveUserDelegate* delegate) = 0;
 
   // Removes the user from the persistent list only. Also removes the user's
   // picture.
-  virtual void RemoveUserFromList(const std::string& user_id) = 0;
+  virtual void RemoveUserFromList(const UserID& user_id) = 0;
 
   // Returns true if a user with the given user id is found in the persistent
   // list or currently logged in as ephemeral.
-  virtual bool IsKnownUser(const std::string& user_id) const = 0;
+  virtual bool IsKnownUser(const UserID& user_id) const = 0;
 
   // Returns the user with the given user id if found in the persistent
   // list or currently logged in as ephemeral. Returns |NULL| otherwise.
-  virtual const User* FindUser(const std::string& user_id) const = 0;
+  virtual const User* FindUser(const UserID& user_id) const = 0;
 
   // Returns the user with the given user id if found in the persistent
   // list or currently logged in as ephemeral. Returns |NULL| otherwise.
   // Same as FindUser but returns non-const pointer to User object.
-  virtual User* FindUserAndModify(const std::string& user_id) = 0;
+  virtual User* FindUserAndModify(const UserID& user_id) = 0;
 
   // Returns the logged-in user.
   // TODO(nkostylev): Deprecate this call, move clients to GetActiveUser().
   // http://crbug.com/230852
   virtual const User* GetLoggedInUser() const = 0;
   virtual User* GetLoggedInUser() = 0;
 
   // Returns the logged-in user that is currently active within this session.
   // There could be multiple users logged in at the the same but for now
   // we support only one of them being active.
   virtual const User* GetActiveUser() const = 0;
   virtual User* GetActiveUser() = 0;
 
   // Returns the primary user of the current session. It is recorded for the
   // first signed-in user and does not change thereafter.
   virtual const User* GetPrimaryUser() const = 0;
 
   // Saves user's oauth token status in local state preferences.
   virtual void SaveUserOAuthStatus(
-      const std::string& user_id,
+      const UserID& user_id,
       User::OAuthTokenStatus oauth_token_status) = 0;
 
   // Saves a flag indicating whether online authentication against GAIA should
   // be enforced during the user's next sign-in.
-  virtual void SaveForceOnlineSignin(const std::string& user_id,
+  virtual void SaveForceOnlineSignin(const UserID& user_id,
                                      bool force_online_signin) = 0;
 
   // Saves user's displayed name in local state preferences.
   // Ignored If there is no such user.
-  virtual void SaveUserDisplayName(const std::string& user_id,
+  virtual void SaveUserDisplayName(const UserID& user_id,
                                    const base::string16& display_name) = 0;
 
   // Updates data upon User Account download.
-  virtual void UpdateUserAccountData(const std::string& user_id,
+  virtual void UpdateUserAccountData(const UserID& user_id,
                                      const UserAccountData& account_data) = 0;
 
   // Returns the display name for user |user_id| if it is known (was
   // previously set by a |SaveUserDisplayName| call).
   // Otherwise, returns an empty string.
   virtual base::string16 GetUserDisplayName(
-      const std::string& user_id) const = 0;
+      const UserID& user_id) const = 0;
 
   // Saves user's displayed (non-canonical) email in local state preferences.
   // Ignored If there is no such user.
-  virtual void SaveUserDisplayEmail(const std::string& user_id,
+  virtual void SaveUserDisplayEmail(const UserID& user_id,
                                     const std::string& display_email) = 0;
 
   // Returns the display email for user |user_id| if it is known (was
   // previously set by a |SaveUserDisplayEmail| call).
   // Otherwise, returns |user_id| itself.
-  virtual std::string GetUserDisplayEmail(const std::string& user_id) const = 0;
+  virtual std::string GetUserDisplayEmail(const UserID& user_id) const = 0;
 
   // Saves user's type for user |user_id| into local state preferences.
   // Ignored If there is no such user.
-  virtual void SaveUserType(const std::string& user_id,
+  virtual void SaveUserType(const UserID& user_id,
                             const UserType& user_type) = 0;
 
   // Returns true if current user is an owner.
   virtual bool IsCurrentUserOwner() const = 0;
 
   // Returns true if current user is not existing one (hasn't signed in before).
   virtual bool IsCurrentUserNew() const = 0;
 
   // Returns true if data stored or cached for the current user outside that
   // user's cryptohome (wallpaper, avatar, OAuth token status, display name,
@@ skipping 30 matching lines @@
 
   // Returns true if we're logged in and browser has been started i.e.
   // browser_creator.LaunchBrowser(...) was called after sign in
   // or restart after crash.
   virtual bool IsSessionStarted() const = 0;
 
   // Returns true if data stored or cached for the user with the given user id
   // address outside that user's cryptohome (wallpaper, avatar, OAuth token
   // status, display name, display email) is to be treated as ephemeral.
   virtual bool IsUserNonCryptohomeDataEphemeral(
-      const std::string& user_id) const = 0;
+      const UserID& user_id) const = 0;
 
   virtual void AddObserver(Observer* obs) = 0;
   virtual void RemoveObserver(Observer* obs) = 0;
 
   virtual void AddSessionStateObserver(UserSessionStateObserver* obs) = 0;
   virtual void RemoveSessionStateObserver(UserSessionStateObserver* obs) = 0;
 
   virtual void NotifyLocalStateChanged() = 0;
 
   // Changes the child status and notifies observers.
@@ skipping 102 matching lines @@
   static UserManager* GetForTesting();
 
   // Sets UserManager instance to the given |user_manager|.
   // Returns the previous value of the instance.
   static UserManager* SetForTesting(UserManager* user_manager);
 };
 
 }  // namespace user_manager
 
 #endif  // COMPONENTS_USER_MANAGER_USER_MANAGER_H_