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

chrome/browser/chromeos/login/screens/user_selection_screen.h

Index: chrome/browser/chromeos/login/screens/user_selection_screen.h
diff --git a/chrome/browser/chromeos/login/screens/user_selection_screen.h b/chrome/browser/chromeos/login/screens/user_selection_screen.h
index 7b9c684e59acae880f6298c8478ec588b96578d0..6e0d29ff8a1519df7fdf92e302419d8175744460 100644
--- a/chrome/browser/chromeos/login/screens/user_selection_screen.h
+++ b/chrome/browser/chromeos/login/screens/user_selection_screen.h
@@ -42,20 +42,20 @@ class UserSelectionScreen
 
   static const user_manager::UserList PrepareUserListForSending(
       const user_manager::UserList& users,
-      std::string owner,
+      const user_manager::UserID& owner,
       bool is_signin_to_add);
 
 
   virtual void Init(const user_manager::UserList& users, bool show_guest);
   const user_manager::UserList& GetUsers() const;
-  void OnUserImageChanged(const user_manager::User& user);
-  void OnBeforeUserRemoved(const std::string& username);
-  void OnUserRemoved(const std::string& username);
+  void OnUserImageChanged(const user_manager::User& user_id);
+  void OnBeforeUserRemoved(const user_manager::UserID& user_id);
+  void OnUserRemoved(const user_manager::UserID& user_id);
 
   void OnPasswordClearTimerExpired();
 
   void HandleGetUsers();
-  void CheckUserStatus(const std::string& user_id);
+  void CheckUserStatus(const user_manager::UserID& user_id);
 
   // ui::UserActivityDetector implementation:
   void OnUserActivity(const ui::Event* event) override;
@@ -65,28 +65,28 @@ class UserSelectionScreen
   // proximity_auth::ScreenlockBridge::LockHandler implementation:
   void ShowBannerMessage(const base::string16& message) override;
   void ShowUserPodCustomIcon(
-      const std::string& user_email,
+      const user_manager::UserID& user_id,
       const proximity_auth::ScreenlockBridge::UserPodCustomIconOptions& icon)
       override;
-  void HideUserPodCustomIcon(const std::string& user_email) override;
+  void HideUserPodCustomIcon(const user_manager::UserID& user_id) override;
 
   void EnableInput() override;
-  void SetAuthType(const std::string& user_email,
+  void SetAuthType(const user_manager::UserID& user_id,
                    AuthType auth_type,
                    const base::string16& auth_value) override;
-  AuthType GetAuthType(const std::string& user_email) const override;
+  AuthType GetAuthType(const user_manager::UserID& user_id) const override;
   ScreenType GetScreenType() const override;
 
-  void Unlock(const std::string& user_email) override;
-  void AttemptEasySignin(const std::string& user_email,
+  void Unlock(const user_manager::UserID& user_id) override;
+  void AttemptEasySignin(const user_manager::UserID& user_id,
                          const std::string& secret,
                          const std::string& key_label) override;
 
   // UserBoardModel implementation.
   void SendUserList() override;
-  void HardLockPod(const std::string& user_id) override;
-  void AttemptEasyUnlock(const std::string& user_id) override;
-  void RecordClickOnLockIcon(const std::string& user_id) override;
+  void HardLockPod(const user_manager::UserID& user_id) override;
+  void AttemptEasyUnlock(const user_manager::UserID& user_id) override;
+  void RecordClickOnLockIcon(const user_manager::UserID& user_id) override;
 
   // Fills |user_dict| with information about |user|.
   static void FillUserDictionary(
@@ -115,13 +115,13 @@ class UserSelectionScreen
   UserBoardView* view_;
 
   // Map from public session user IDs to recommended locales set by policy.
-  typedef std::map<std::string, std::vector<std::string> >
+  typedef std::map<user_manager::UserID, std::vector<std::string> >
       PublicSessionRecommendedLocaleMap;
   PublicSessionRecommendedLocaleMap public_session_recommended_locales_;
 
  private:
   EasyUnlockService* GetEasyUnlockServiceForUser(
-      const std::string& user_id) const;
+      const user_manager::UserID& user_id) const;
 
   void OnUserStatusChecked(const user_manager::UserID& user_id,
                            TokenHandleUtil::TokenHandleStatus status);
@@ -137,7 +137,7 @@ class UserSelectionScreen
 
   // Map of usernames to their current authentication type. If a user is not
   // contained in the map, it is using the default authentication type.
-  std::map<std::string, proximity_auth::ScreenlockBridge::LockHandler::AuthType>
+  std::map<user_manager::UserID, proximity_auth::ScreenlockBridge::LockHandler::AuthType>
       user_auth_type_map_;
 
   // Timer for measuring idle state duration before password clear.