Fix calculation of live-bytes in pages.

[Deleted, and reuploaded as http://codereview.chromium.org/7970009/ but relative 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.