Stay in fast enum case if the empty_slow_element_dictionary is used.
This fixes slow for-in over frozen objects.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/135903014

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18705 ce2b1a6d-e550-0410-aec6-3dcde31c8c00