The root visitor now collects marked roots in the marking worklist and
filters out objects that are not in the new space.

This reduces average marking time in MinorMC in Richards from 0.08ms
to 0.04ms:

baseline mark:
  len: 22
  min: 0.07
  max: 0.18
  avg: 0.0809090909091
  [0,5[: 22

mark
  len: 22
  min: 0.03
  max: 0.13
  avg: 0.0409090909091
  [0,5[: 22

Bug: chromium:651354
Change-Id: I979e2f5ba331f88029b69bab23978f7fcadb7024
Reviewed-on: https://chromium-review.googlesource.com/1055490
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53128}