Add a mojo interface for AccountId

ash/mus/test/ash_test_impl_mus.cc

Index: ash/mus/test/ash_test_impl_mus.cc
diff --git a/ash/mus/test/ash_test_impl_mus.cc b/ash/mus/test/ash_test_impl_mus.cc
index 043a4ed52010e8af3b3a007cff2002326adaddfc..909c2c61c5da8e47574cf2b285b662be00fd43a3 100644
--- a/ash/mus/test/ash_test_impl_mus.cc
+++ b/ash/mus/test/ash_test_impl_mus.cc
@@ -115,7 +115,7 @@ void AshTestImplMus::SimulateUserLogin() {
   session->session_id = 1;
   session->type = user_manager::USER_TYPE_REGULAR;
   const std::string email("ash.user@example.com");
-  session->serialized_account_id = AccountId::FromUserEmail(email).Serialize();
+  session->account_id = AccountId::FromUserEmail(email);
   session->display_name = "Ash User";
   session->display_email = email;
   session_controller->UpdateUserSession(std::move(session));