Fix heuristic triggering incremental marking.

Fix heuristic triggering incremental marking.

This changes the heuristic that starts incremental marking to be based
on a more accurate heap size estimation. Pages being swept lazily can be
accounted using the live bytes counter.

R=mstarzinger@chromium.org
BUG=v8:1682
Committed: https://code.google.com/p/v8/source/detail?r=11004

[Yang, 2012-03-09 22:04:20]

PTAL.

[Yang, 2012-03-11 09:08:26]

On 2012/03/09 22:04:20, Yang wrote:
> PTAL.

Uodated description as suggested.

[Michael Starzinger, 2012-03-12 08:40:54]

LGTM.