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

chrome/browser/chromeos/login/ui/models/user_board_model.h

Index: chrome/browser/chromeos/login/ui/models/user_board_model.h
diff --git a/chrome/browser/chromeos/login/ui/models/user_board_model.h b/chrome/browser/chromeos/login/ui/models/user_board_model.h
index 757e55847eb7adf46bee886d82ab72d56ae8acba..e7e35cf33302472394c7e32d9395af78cf57f16e 100644
--- a/chrome/browser/chromeos/login/ui/models/user_board_model.h
+++ b/chrome/browser/chromeos/login/ui/models/user_board_model.h
@@ -7,6 +7,10 @@
 
 #include "chrome/browser/chromeos/login/screens/base_screen.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class UserBoardModel : public BaseScreen {
@@ -18,9 +22,9 @@ class UserBoardModel : public BaseScreen {
   virtual void SendUserList() = 0;
 
   // Methods for easy unlock support.
-  virtual void HardLockPod(const std::string& user_id) = 0;
-  virtual void AttemptEasyUnlock(const std::string& user_id) = 0;
-  virtual void RecordClickOnLockIcon(const std::string& user_id) = 0;
+  virtual void HardLockPod(const user_manager::UserID& user_id) = 0;
+  virtual void AttemptEasyUnlock(const user_manager::UserID& user_id) = 0;
+  virtual void RecordClickOnLockIcon(const user_manager::UserID& user_id) = 0;
 
   // BaseScreen implementation:
   std::string GetName() const override;