Was only used when concurrent marking was disabled at compile-time.

Change-Id: I3977a2cbcfb27e527b2bca4b49887558595c4215
Reviewed-on: https://chromium-review.googlesource.com/1128959Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#54384}