The flag is --parallel-marking.

Bug: chromium:750084
Change-Id: I20ab5945d2cc41d44b29d7090a3436d028588540
Reviewed-on: https://chromium-review.googlesource.com/756709Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49178}