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

Port r21457 (8db39a8)

Original commit message:
This fixes inobject slack tracking for prototype inheritance pattern that uses closures.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/294973013

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21470 ce2b1a6d-e550-0410-aec6-3dcde31c8c00