arc: Partially migrate mojo types

chrome/browser/chromeos/arc/arc_auth_service.cc

Index: chrome/browser/chromeos/arc/arc_auth_service.cc
diff --git a/chrome/browser/chromeos/arc/arc_auth_service.cc b/chrome/browser/chromeos/arc/arc_auth_service.cc
index d9484ce7b209ab75da1c1bc269def940843cad84..a342fc0878629bbb990c39a4211cd71a78bb7bc8 100644
--- a/chrome/browser/chromeos/arc/arc_auth_service.cc
+++ b/chrome/browser/chromeos/arc/arc_auth_service.cc
@@ -164,7 +164,7 @@ class ArcAuthService::AccountInfoNotifier {
         DCHECK(!account_info_callback_.is_null());
         mojom::AccountInfoPtr account_info = mojom::AccountInfo::New();
         if (!is_enforced) {
-          account_info->auth_code = nullptr;
+          account_info->auth_code = base::nullopt;
         } else {
           account_info->auth_code = auth_code;
         }