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

chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.h

Index: chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.h
diff --git a/chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.h b/chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.h
index 1c85437018562464c46c1797ba959bec7007482c..54673a4df0d0da7d78b78bdf30fa4ea869895b48 100644
--- a/chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.h
+++ b/chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.h
@@ -11,6 +11,10 @@
 #include "chrome/browser/chromeos/login/ui/views/user_board_view.h"
 #include "chrome/browser/ui/webui/chromeos/login/base_screen_handler.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class SigninScreenHandler;
@@ -32,23 +36,23 @@ class UserBoardScreenHandler : public BaseScreenHandler, public UserBoardView {
 
   // Handlers
   void HandleGetUsers();
-  void HandleHardlockPod(const std::string& user_id);
-  void HandleAttemptUnlock(const std::string& user_id);
-  void HandleRecordClickOnLockIcon(const std::string& user_id);
+  void HandleHardlockPod(const std::string& user_email);
+  void HandleAttemptUnlock(const std::string& user_email);
+  void HandleRecordClickOnLockIcon(const std::string& user_email);
 
   // UserBoardView implementation:
-  void SetPublicSessionDisplayName(const std::string& user_id,
+  void SetPublicSessionDisplayName(const user_manager::UserID& user_id,
                                    const std::string& display_name) override;
-  void SetPublicSessionLocales(const std::string& user_id,
+  void SetPublicSessionLocales(const user_manager::UserID& user_id,
                                scoped_ptr<base::ListValue> locales,
                                const std::string& default_locale,
                                bool multiple_recommended_locales) override;
   void ShowBannerMessage(const base::string16& message) override;
-  void ShowUserPodCustomIcon(const std::string& user_id,
+  void ShowUserPodCustomIcon(const user_manager::UserID& user_id,
                              const base::DictionaryValue& icon) override;
-  void HideUserPodCustomIcon(const std::string& user_id) override;
+  void HideUserPodCustomIcon(const user_manager::UserID& user_id) override;
   void SetAuthType(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       proximity_auth::ScreenlockBridge::LockHandler::AuthType auth_type,
       const base::string16& initial_value) override;