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

chrome/browser/extensions/api/screenlock_private/screenlock_private_api.h

 // Copyright 2013 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_EXTENSIONS_API_SCREENLOCK_PRIVATE_SCREENLOCK_PRIVATE_API_H_
 #define CHROME_BROWSER_EXTENSIONS_API_SCREENLOCK_PRIVATE_SCREENLOCK_PRIVATE_API_H_
 
 #include <string>
 
 #include "chrome/browser/extensions/chrome_extension_function.h"
@@ skipping 55 matching lines @@
       ScreenlockPrivateEventRouter>*
       GetFactoryInstance();
   void Shutdown() override;
 
   // proximity_auth::ScreenlockBridge::Observer
   void OnScreenDidLock(proximity_auth::ScreenlockBridge::LockHandler::ScreenType
                            screen_type) override;
   void OnScreenDidUnlock(
       proximity_auth::ScreenlockBridge::LockHandler::ScreenType screen_type)
       override;
-  void OnFocusedUserChanged(const std::string& user_id) override;
+  void OnFocusedUserChanged(const user_manager::UserID& user_id) override;
 
  private:
   friend class extensions::BrowserContextKeyedAPIFactory<
       ScreenlockPrivateEventRouter>;
 
   // BrowserContextKeyedAPI
   static const char* service_name() {
     return "ScreenlockPrivateEventRouter";
   }
   static const bool kServiceIsNULLWhileTesting = true;
   static const bool kServiceRedirectedInIncognito = true;
 
   void DispatchEvent(const std::string& event_name, base::Value* arg);
 
   content::BrowserContext* browser_context_;
   DISALLOW_COPY_AND_ASSIGN(ScreenlockPrivateEventRouter);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_SCREENLOCK_PRIVATE_SCREENLOCK_PRIVATE_API_H_