Unbreak readline support.

Things are still far from being nice, the editor registration/handling in d8 is
still embarrassing. Nevertheless things work with readline support again. Fixed
a missing Locker on the way.

TBR=adamk@chromium.org

Review URL: https://codereview.chromium.org/12494010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13909 ce2b1a6d-e550-0410-aec6-3dcde31c8c00