Inobject slack tracking is done on a per-closure basis instead of per-shared info basis.
This fixes inobject slack tracking for prototype inheritance pattern that uses closures.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/283383006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21436 ce2b1a6d-e550-0410-aec6-3dcde31c8c00