r5147 wrongly assumed that a code cache for a slow case map is always empty.
This patch solves this: whenever we attempt to add a stub to a map's code cache 
we check that this map is cached. If it is we give the object its own copy
of the map and only then modify the map.

Review URL: http://codereview.chromium.org/3134027

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5342 ce2b1a6d-e550-0410-aec6-3dcde31c8c00