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

chrome/browser/chromeos/login/reauth_stats.h

Index: chrome/browser/chromeos/login/reauth_stats.h
diff --git a/chrome/browser/chromeos/login/reauth_stats.h b/chrome/browser/chromeos/login/reauth_stats.h
index 99bd159e92c6b5e18c2d82569428448b7b705834..f3a957b5bda755fb8a48179c4a860669f2b7f431 100644
--- a/chrome/browser/chromeos/login/reauth_stats.h
+++ b/chrome/browser/chromeos/login/reauth_stats.h
@@ -7,6 +7,10 @@
 
 #include <string>
 
+namespace user_manager {
+class UserID;
+} // namespace user_manager
+
 namespace chromeos {
 
 // Track all the ways a user may be sent through the re-auth flow.
@@ -51,8 +55,8 @@ enum ReauthReason {
   NUM_REAUTH_FLOW_REASONS,
 };
 
-void RecordReauthReason(const std::string& user_id, ReauthReason reason);
-void SendReauthReason(const std::string& user_id);
+void RecordReauthReason(const user_manager::UserID& user_id, ReauthReason reason);
+void SendReauthReason(const user_manager::UserID& user_id);
 
 }  // namespace chromeos