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

chrome/browser/chromeos/policy/device_local_account_policy_service.cc

Index: chrome/browser/chromeos/policy/device_local_account_policy_service.cc
diff --git a/chrome/browser/chromeos/policy/device_local_account_policy_service.cc b/chrome/browser/chromeos/policy/device_local_account_policy_service.cc
index f93d6379d3a8130fe886385fc29459764078b8bd..423e35f5d7a6779f7252865131b750314fa8e051 100644
--- a/chrome/browser/chromeos/policy/device_local_account_policy_service.cc
+++ b/chrome/browser/chromeos/policy/device_local_account_policy_service.cc
@@ -314,7 +314,7 @@ void DeviceLocalAccountPolicyService::Connect(
 
 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;
@@ -323,7 +323,7 @@ DeviceLocalAccountPolicyBroker*
 }
 
 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();
 }
@@ -577,7 +577,7 @@ DeviceLocalAccountPolicyBroker*
 }
 
 void DeviceLocalAccountPolicyService::NotifyPolicyUpdated(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   FOR_EACH_OBSERVER(Observer, observers_, OnPolicyUpdated(user_id));
 }