Drive-by-fix: map_set() must return a pointer to the UniqueSet
instead of a copy.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/244383002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20870 ce2b1a6d-e550-0410-aec6-3dcde31c8c00