Revert "Inobject slack tracking is done on a per-closure basis instead of per-shared info basis."

This reverts r21436.

TBR=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/295933012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21437 ce2b1a6d-e550-0410-aec6-3dcde31c8c00