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

chrome/browser/chromeos/login/screens/chrome_user_selection_screen.cc

Index: chrome/browser/chromeos/login/screens/chrome_user_selection_screen.cc
diff --git a/chrome/browser/chromeos/login/screens/chrome_user_selection_screen.cc b/chrome/browser/chromeos/login/screens/chrome_user_selection_screen.cc
index 37737c7c8bc000704fa056ca8d60a21ae5bdf0d5..153ac3565096168d607973854f3b4d286ed97efa 100644
--- a/chrome/browser/chromeos/login/screens/chrome_user_selection_screen.cc
+++ b/chrome/browser/chromeos/login/screens/chrome_user_selection_screen.cc
@@ -22,6 +22,7 @@
 #include "components/policy/core/common/policy_map.h"
 #include "components/policy/core/common/policy_types.h"
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 #include "components/user_manager/user_manager.h"
 #include "components/user_manager/user_type.h"
 #include "policy/policy_constants.h"
@@ -59,7 +60,7 @@ void ChromeUserSelectionScreen::SendUserList() {
   handler_initialized_ = true;
 }
 
-void ChromeUserSelectionScreen::OnPolicyUpdated(const std::string& user_id) {
+void ChromeUserSelectionScreen::OnPolicyUpdated(const user_manager::UserID& user_id) {
   policy::DeviceLocalAccountPolicyBroker* broker =
       device_local_account_policy_service_->GetBrokerForUser(user_id);
   if (!broker)
@@ -76,7 +77,7 @@ void ChromeUserSelectionScreen::OnDeviceLocalAccountsChanged() {
 
 void ChromeUserSelectionScreen::CheckForPublicSessionDisplayNameChange(
     policy::DeviceLocalAccountPolicyBroker* broker) {
-  const std::string& user_id = broker->user_id();
+  const user_manager::UserID& user_id = broker->user_id();
   const std::string& display_name = broker->GetDisplayName();
   if (display_name == public_session_display_names_[user_id])
     return;
@@ -106,7 +107,7 @@ void ChromeUserSelectionScreen::CheckForPublicSessionDisplayNameChange(
 
 void ChromeUserSelectionScreen::CheckForPublicSessionLocalePolicyChange(
     policy::DeviceLocalAccountPolicyBroker* broker) {
-  const std::string& user_id = broker->user_id();
+  const user_manager::UserID& user_id = broker->user_id();
   const policy::PolicyMap::Entry* entry =
       broker->core()->store()->policy_map().Get(policy::key::kSessionLocales);
 
@@ -142,7 +143,7 @@ void ChromeUserSelectionScreen::CheckForPublicSessionLocalePolicyChange(
 }
 
 void ChromeUserSelectionScreen::SetPublicSessionDisplayName(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   const user_manager::User* user =
       user_manager::UserManager::Get()->FindUser(user_id);
   if (!user || user->GetType() != user_manager::USER_TYPE_PUBLIC_ACCOUNT)
@@ -153,7 +154,7 @@ void ChromeUserSelectionScreen::SetPublicSessionDisplayName(
 }
 
 void ChromeUserSelectionScreen::SetPublicSessionLocales(
-    const std::string& user_id,
+    const user_manager::UserID& user_id,
     const std::vector<std::string>& recommended_locales) {
   if (!handler_initialized_)
     return;