Merged from GC branch to bleeding_edge

Fix calculation of live-bytes in pages.

The "live bytes" count is *really* a "marked black" count - i.e., the count of bytes *known* to be live.

Fix aggravating bug on X64 where assembler code used a value that was off
by a factor of 2^31.

Ensure that sweeping clears live-bytes. Added other missing increments.

Added print statements to trace live-byte modifications, under a flag.

Still a few cases of undercounting left.

(New issue to merge from GC branch to bleeding_edge)

Committed: http://code.google.com/p/v8/source/detail?r=9338

[Lasse Reichstein, 2011-09-20 09:40:22]

PTAL

[Vyacheslav Egorov (Chromium), 2011-09-20 10:43:18]

lgtm

please try to reenable tests in mjsunit.status