Implement shrinking of paged spaces during sweeping.

Implement shrinking of paged spaces during sweeping.

For each paged space we release all but one of the unused pages after
marking (when we know the number of live bytes) but before actually
sweeping it. This is not yet done for lazy swept pages.

R=erik.corry@gmail.com
BUG=v8:1614
Committed: http://code.google.com/p/v8/source/detail?r=9481

[Michael Starzinger, 2011-09-13 17:12:13]

PTAL.

[Erik Corry, 2011-09-14 08:06:13]

LGTM

I like changes that delete code :-)

[Michael Starzinger, 2011-09-16 11:23:20]

Just for the record: This is blocked by issue v8:1614 because it depends on
page->LiveBytes() being (semi-)accurate. So I am holding off landing it until
that is resolved.