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

chrome/browser/chromeos/policy/device_local_account_policy_service.cc

 // Copyright (c) 2012 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/chromeos/policy/device_local_account_policy_service.h"
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 296 matching lines @@
   for (PolicyBrokerMap::iterator it(policy_brokers_.begin());
        it != policy_brokers_.end(); ++it) {
     it->second->ConnectIfPossible(device_settings_service_,
                                   device_management_service_,
                                   request_context_);
   }
 }
 
 DeviceLocalAccountPolicyBroker*
     DeviceLocalAccountPolicyService::GetBrokerForUser(
-        const std::string& user_id) {
+        const user_manager::UserID& user_id) {
   PolicyBrokerMap::iterator entry = policy_brokers_.find(user_id);
   if (entry == policy_brokers_.end())
     return nullptr;
 
   return entry->second;
 }
 
 bool DeviceLocalAccountPolicyService::IsPolicyAvailableForUser(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   DeviceLocalAccountPolicyBroker* broker = GetBrokerForUser(user_id);
   return broker && broker->core()->store()->is_managed();
 }
 
 void DeviceLocalAccountPolicyService::AddObserver(Observer* observer) {
   observers_.AddObserver(observer);
 }
 
 void DeviceLocalAccountPolicyService::RemoveObserver(Observer* observer) {
   observers_.RemoveObserver(observer);
@@ skipping 233 matching lines @@
         CloudPolicyStore* store) {
   for (PolicyBrokerMap::iterator it(policy_brokers_.begin());
        it != policy_brokers_.end(); ++it) {
     if (it->second->core()->store() == store)
       return it->second;
   }
   return nullptr;
 }
 
 void DeviceLocalAccountPolicyService::NotifyPolicyUpdated(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   FOR_EACH_OBSERVER(Observer, observers_, OnPolicyUpdated(user_id));
 }
 
 }  // namespace policy