The scavenger should never consider mark bits for promotion/copy as this creates
weird livetimes at the start of incremental marking. E.g. consider an object
marked black by the marker at the start of incremental marking. A scavenge would
promote it to the old generation although it could --and for short-living
objects actually does-- become unreachable during marking

Also, keeping this invariant significantly simplifies young generation mark
compacting as we can compare against the scavenging decision without keeping
different sets of markbits.

BUG=chromium:651354
R=hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2397713002
Cr-Commit-Position: refs/heads/master@{#40026}