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

components/proximity_auth/ble/proximity_auth_ble_system.cc

 // 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.
 
 #include "components/proximity_auth/ble/proximity_auth_ble_system.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "components/proximity_auth/ble/bluetooth_low_energy_connection.h"
 #include "components/proximity_auth/ble/bluetooth_low_energy_connection_finder.h"
@@ skipping 59 matching lines @@
 void ProximityAuthBleSystem::OnScreenDidUnlock(
     ScreenlockBridge::LockHandler::ScreenType screen_type) {
   VLOG(1) << "OnScreenDidUnlock: " << screen_type;
   if (connection_)
     connection_->Disconnect();
 
   connection_.reset();
   connection_finder_.reset();
 };
 
-void ProximityAuthBleSystem::OnFocusedUserChanged(const std::string& user_id) {
-  VLOG(1) << "OnFocusedUserChanged: " << user_id;
+void ProximityAuthBleSystem::OnFocusedUserChanged(const user_manager::UserID& user_id) {
+  VLOG(1) << "OnFocusedUserChanged: " << user_id.Serialize();
 };
 
 void ProximityAuthBleSystem::OnConnectionFound(
     scoped_ptr<Connection> connection) {
   VLOG(1) << "Connection found. Unlock.";
 
   connection_ = connection.Pass();
 
   // Unlock the screen when a connection is found.
   //
   // Note that this magically unlocks Chrome (no user interaction is needed).
   // This user experience for this operation will be greately improved once
   // the Proximity Auth Unlock Manager migration to C++ is done.
   screenlock_bridge_->Unlock(browser_context_);
 }
 
 }  // namespace proximity_auth