- Only mark a single bit (grey)
- Increment live bytes after visiting, avoiding the map lookup for size
  in ObjectMarking

Raw speed improvements should be around 20%-30%.

Bug: chromium:651354
Change-Id: Ib58d1aee0b99d8e628a0191f90a2ffad9324b915
Reviewed-on: https://chromium-review.googlesource.com/509548
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45484}