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)

Review URL: http://codereview.chromium.org/7970009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9338 ce2b1a6d-e550-0410-aec6-3dcde31c8c00