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

chrome/browser/chromeos/policy/device_local_account.h

Index: chrome/browser/chromeos/policy/device_local_account.h
diff --git a/chrome/browser/chromeos/policy/device_local_account.h b/chrome/browser/chromeos/policy/device_local_account.h
index 88271192ab4c6c2393bbc7c51ccaa36ddcae60d3..71de8e86ebb2d503fd5d6f3b7ad70047679de375 100644
--- a/chrome/browser/chromeos/policy/device_local_account.h
+++ b/chrome/browser/chromeos/policy/device_local_account.h
@@ -8,6 +8,8 @@
 #include <string>
 #include <vector>
 
+#include "components/user_manager/user_id.h"
+
 namespace chromeos {
 class CrosSettings;
 class OwnerSettingsServiceChromeOS;
@@ -39,9 +41,10 @@ struct DeviceLocalAccount {
   //   account. The only constraints are that the |account_id| be unique and,
   //   for legacy reasons, it contain an @ symbol.
   // * The |user_id| is a synthesized identifier that is guaranteed to be
-  //   unique, contain an @ symbol, not collide with the |user_id| of any other
-  //   user on the device (such as regular users or supervised users) and be
-  //   identifiable as belonging to a device-local account by.
+  //   unique, which email contains an @ symbol, doesn not collide with the
+  //   |user_id| of any other user on the device (such as regular users or
+  //   supervised users) and be identifiable as belonging to a device-local
+  //   account by.
   // The |account_id| is primarily used by policy code: If device policy defines
   // a device-local account with a certain |account_id|, the user policy for
   // that account has to be fetched by referencing the same |account_id|.
@@ -52,17 +55,17 @@ struct DeviceLocalAccount {
   // another |user_id| on the device and cannot be easily identified as
   // belonging to a device-local account.
   std::string account_id;
-  std::string user_id;
+  user_manager::UserID user_id;
   std::string kiosk_app_id;
   std::string kiosk_app_update_url;
 };
 
-std::string GenerateDeviceLocalAccountUserId(const std::string& account_id,
+user_manager::UserID GenerateDeviceLocalAccountUserId(const std::string& account_id,
                                              DeviceLocalAccount::Type type);
 
 // Determines whether |user_id| belongs to a device-local account and if so,
 // returns the type of device-local account in |type| unless |type| is NULL.
-bool IsDeviceLocalAccountUser(const std::string& user_id,
+bool IsDeviceLocalAccountUser(const user_manager::UserID& user_id,
                               DeviceLocalAccount::Type* type);
 
 // Stores a list of device-local accounts in |service|. The accounts are stored