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).


Committed: http://code.google.com/p/v8/source/detail?r=5571

[Vitaly Repeshko, 2010-09-30 17:36:39]

LGTM