MIPS: Fix r21470 “Reland r21442 Inobject slack tracking is done on a per-closure basis instead of per-shared info basis."

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/297093002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21471 ce2b1a6d-e550-0410-aec6-3dcde31c8c00