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

components/proximity_auth/cryptauth/cryptauth_enrollment_utils.h

 // Copyright 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 COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ENROLLMENT_UTILS_H
 #define COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ENROLLMENT_UTILS_H
 
 #include <string>
 
+namespace user_manager {
+class UserID;
+}
+
 namespace proximity_auth {
 
 // Calculates an id for the specified user on the specified device.
 std::string CalculateDeviceUserId(const std::string& device_id,
-                                  const std::string& user_id);
+                                  const user_manager::UserID& user_id);
 
 }  // namespace proximity_auth
 
 #endif  // COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ENROLLMENT_UTILS_H