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

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

 // 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.
 
 #include "chrome/browser/extensions/api/screenlock_private/screenlock_private_api.h"
 
 #include "base/lazy_instance.h"
 #include "base/values.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/signin/easy_unlock_service.h"
@@ skipping 106 matching lines @@
       new base::FundamentalValue(true));
 }
 
 void ScreenlockPrivateEventRouter::OnScreenDidUnlock(
     proximity_auth::ScreenlockBridge::LockHandler::ScreenType screen_type) {
   DispatchEvent(screenlock::OnChanged::kEventName,
       new base::FundamentalValue(false));
 }
 
 void ScreenlockPrivateEventRouter::OnFocusedUserChanged(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
 }
 
 void ScreenlockPrivateEventRouter::DispatchEvent(
     const std::string& event_name,
     base::Value* arg) {
   scoped_ptr<base::ListValue> args(new base::ListValue());
   if (arg)
     args->Append(arg);
   scoped_ptr<extensions::Event> event(new extensions::Event(
       event_name, args.Pass()));
@@ skipping 25 matching lines @@
   args->AppendString(screenlock::ToString(FromLockHandlerAuthType(auth_type)));
   args->AppendString(value);
 
   scoped_ptr<extensions::Event> event(new extensions::Event(
       screenlock::OnAuthAttempted::kEventName, args.Pass()));
   router->BroadcastEvent(event.Pass());
   return true;
 }
 
 }  // namespace extensions