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

components/proximity_auth/ble/proximity_auth_ble_system.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_BLE_PROXIMITY_AUTH_BLE_SYSTEM_H_
 #define COMPONENTS_PROXIMITY_AUTH_BLE_PROXIMITY_AUTH_BLE_SYSTEM_H_
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "components/proximity_auth/screenlock_bridge.h"
 
 namespace content {
 class BrowserContext;
 }
 
 namespace device {
 class BluetoothGattConnection;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace proximity_auth {
 
 class BluetoothLowEnergyConnection;
 class BluetoothLowEnergyConnectionFinder;
 class Connection;
 
 // This is the main entry point to start Proximity Auth over Bluetooth Low
 // Energy. This is the underlying system for the Smart Lock features. It will
 // discover Bluetooth Low Energy phones and unlock the lock screen if the phone
 // passes an authorization and authentication protocol.
 class ProximityAuthBleSystem : public ScreenlockBridge::Observer {
  public:
   ProximityAuthBleSystem(ScreenlockBridge* screenlock_bridge,
                          content::BrowserContext* browser_context);
   ~ProximityAuthBleSystem() override;
 
   // ScreenlockBridge::Observer:
   void OnScreenDidLock(
       ScreenlockBridge::LockHandler::ScreenType screen_type) override;
   void OnScreenDidUnlock(
       ScreenlockBridge::LockHandler::ScreenType screen_type) override;
-  void OnFocusedUserChanged(const std::string& user_id) override;
+  void OnFocusedUserChanged(const user_manager::UserID& user_id) override;
 
  private:
   // Handler for a new connection found event.
   void OnConnectionFound(scoped_ptr<Connection> connection);
 
   ScreenlockBridge* screenlock_bridge_;  // Not owned. Must outlive this object.
   content::BrowserContext*
       browser_context_;  // Not owned. Must outlive this object.
 
   scoped_ptr<BluetoothLowEnergyConnectionFinder> connection_finder_;
 
   scoped_ptr<Connection> connection_;
 
   base::WeakPtrFactory<ProximityAuthBleSystem> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ProximityAuthBleSystem);
 };
 
 }  // namespace proximity_auth
 
 #endif  // COMPONENTS_PROXIMITY_AUTH_BLE_PROXIMITY_AUTH_BLE_SYSTEM_H_