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

chromeos/login/user_names.cc

 // Copyright 2014 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 "chromeos/login/user_names.h"
 
+#include "components/user_manager/user_id.h"
 #include "google_apis/gaia/gaia_auth_util.h"
 
+namespace {
+
+// Should match cros constant in platform/libchromeos/chromeos/cryptohome.h
+const char* kGuestUserName = "$guest";
+
+const char* kSignInUser = "sign-in-user-id";
+
+const char* kStubUser = "stub-user@example.com";
+} // anonymous namespace
+
 namespace chromeos {
 
 namespace login {
 
-const char* kStubUser = "stub-user@example.com";
-
-const char* kSignInUser = "sign-in-user-id";
-
-// Should match cros constant in platform/libchromeos/chromeos/cryptohome.h
-const char* kGuestUserName = "$guest";
+const char* kLegacyGuestUserName = kGuestUserName;
 
 const char* kSupervisedUserDomain = "locally-managed.localhost";
 
 std::string CanonicalizeUserID(const std::string& user_id) {
-  if (user_id == chromeos::login::kGuestUserName)
+  if (user_id == kGuestUserName)
     return user_id;
   return gaia::CanonicalizeEmail(user_id);
 }
 
+user_manager::UserID GetSignInUserID() {
+  return user_manager::UserID(std::string() /* gaia_id */, kSignInUser);
+}
+
+user_manager::UserID GetGuestUserID() {
+  return user_manager::UserID(std::string() /* gaia_id */, kGuestUserName);
+}
+
+user_manager::UserID GetStubUserID() {
+  return user_manager::UserID(std::string() /* gaia_id */, kStubUser);
+}
+
 }  // namespace login
 
 }  // namespace chromeos