Fix lazy sweeping heuristics to prevent old-space expansion.

Fix lazy sweeping heuristics to prevent old-space expansion.

This is a temporary fix to prevent excessive expansion of old-space
caused by inaccuracies in lazy sweeper advancing. We actually don't
intend to sweep any pages eagerly, but that requires further changes
to lazy sweeping.

R=ulan@chromium.org,erik.corry@gmail.com
BUG=v8:2194
Committed: https://code.google.com/p/v8/source/detail?r=11904

[Michael Starzinger, 2012-06-22 11:27:54]

Adapted descriptions to reflect that it's a temporary fix. Referenced issue
tracking the upcoming long-term fix.

[ulan, 2012-06-22 11:29:44]

LGTM