Use JSObject::SetMapAndElements rather than set_map + set_elements.

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/218583008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20367 ce2b1a6d-e550-0410-aec6-3dcde31c8c00