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

chrome/browser/chromeos/login/reauth_stats.h

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_CHROMEOS_LOGIN_REAUTH_STATS_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_REAUTH_STATS_H_
 
 #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.
 // This enum is used to define the buckets for an enumerated UMA histogram.
 // Hence, existing enumerated constants should never be reordered, and all new
 // constants should only be appended at the end of the  enumeration.
 enum ReauthReason {
   // Default value: no reauth reasons were detected so far, or the reason was
   // already reported.
   NONE = 0,
@@ skipping 24 matching lines @@
   // etc.
   SYNC_FAILED = 7,
 
   // User cancelled the password change prompt when prompted by Chrome OS.
   PASSWORD_UPDATE_SKIPPED = 8,
 
   // Must be the last value in this list.
   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
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_REAUTH_STATS_H_