This allows us to use the faster write barrier, omitting the
smi check when storing constant heap objects.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/150303002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18947 ce2b1a6d-e550-0410-aec6-3dcde31c8c00