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

chrome/browser/chromeos/settings/cros_settings.h

Index: chrome/browser/chromeos/settings/cros_settings.h
diff --git a/chrome/browser/chromeos/settings/cros_settings.h b/chrome/browser/chromeos/settings/cros_settings.h
index 366e15ca8922f7e900a2ea5103db7f42272071b9..d83f863b3fabe3ac15e21f61ef7d81ba097b8d29 100644
--- a/chrome/browser/chromeos/settings/cros_settings.h
+++ b/chrome/browser/chromeos/settings/cros_settings.h
@@ -22,6 +22,10 @@ class ListValue;
 class Value;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class DeviceSettingsService;
@@ -38,7 +42,7 @@ class CrosSettings : public base::NonThreadSafe {
   // Checks if the given username is whitelisted and allowed to sign-in to
   // this device. |wildcard_match| may be NULL. If it's present, it'll be set to
   // true if the whitelist check was satisfied via a wildcard.
-  static bool IsWhitelisted(const std::string& username, bool* wildcard_match);
+  static bool IsWhitelisted(const user_manager::UserID& user_id, bool* wildcard_match);
 
   // Creates a device settings service instance. This is meant for unit tests,
   // production code uses the singleton returned by Get() above.