Revert part of 11727 as it sometimes tanked V8 benchmark (raytrace)...

Revert part of 11727 as it sometimes tanked V8 benchmark (raytrace) performance for reasons that are not obvious. Now we make objects into fast-case objects when they are made prototypes for other objects, but we do not mark objects that are already fast case with a bit that helps keep them in fast case. Review URL: https://chromiumcodereview.appspot.com/10556004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@11831 ce2b1a6d-e550-0410-aec6-3dcde31c8c00