Use the incremental root marking visitor for overapproximating the weak closure

Use the incremental root marking visitor for overapproximating the weak closure

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}