Check that external references are registered in the serializer.

Inspired by a patch by Slava Chigrin <vchigrin@yandex-team.ru>

R=jkummerow@chromium.org, vchigrin@yandex-team.ru

Review URL: https://codereview.chromium.org/441983002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22896 ce2b1a6d-e550-0410-aec6-3dcde31c8c00