Ignore external strings in old pointer space migration invariant check.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/20142008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15880 ce2b1a6d-e550-0410-aec6-3dcde31c8c00