Since incremental marking hasn't completed yet at this point, we can't
use the MC's root visitor.

Also, we shouldn't hurry with incremental marking while
overapproximating

BUG=v8:3862
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/997423002

Cr-Commit-Position: refs/heads/master@{#27184}