Callers of the ConcurrentMarking API should check the flags, which they were
already mostly doing.

Change-Id: Ie273bb740fe6d2b36dfb924ce813670ebd47bf3f
Reviewed-on: https://chromium-review.googlesource.com/c/1280307Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56638}