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

components/user_manager/user_manager.h

Index: components/user_manager/user_manager.h
diff --git a/components/user_manager/user_manager.h b/components/user_manager/user_manager.h
index 5d9268ea1f1c6cf56c8e7218a600010cb75b98b9..ff3ba2dd074e7ccd2a179485b255921d08d82cc4 100644
--- a/components/user_manager/user_manager.h
+++ b/components/user_manager/user_manager.h
@@ -138,21 +138,21 @@ class USER_MANAGER_EXPORT UserManager {
   // 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).
@@ -170,25 +170,25 @@ class USER_MANAGER_EXPORT UserManager {
   // 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().
@@ -208,42 +208,42 @@ class USER_MANAGER_EXPORT UserManager {
 
   // 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.
@@ -294,7 +294,7 @@ class USER_MANAGER_EXPORT UserManager {
   // 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;