Fine-tune inobject slack tracking.

Fine-tune inobject slack tracking.

The current parameters (initial slack = 6, kGenerousAllocationsCount = 16)
caused 15-20% performance regression on 3d-raytrace on x64
(visible both in standalone shell and in the web harness). This is very
likely some kind of an alignment  problem (it goes away completely with --prof).
A similar problem can be observed on ia32 with initial slack=2.

Increased initial slack to 8 and decreased constructions count to 8.
This made the regression go away (and did not harm any of the other tests).

Review URL: http://codereview.chromium.org/3595002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5571 ce2b1a6d-e550-0410-aec6-3dcde31c8c00