MIPS: Check that external references are registered in the serializer.
MIPS: Check that external references are registered in the serializer.

Port r22896 (4e54978a)

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/444943002

git-svn-id: f98b9d40cb02301bc76fa9e1b46ee668158567fd@22919 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh