Use 1 less task for concurrent marking to allow Oilpan's concurrent
marking a chance to run as well.

Change-Id: I236f78925324dc4d21a38233d75dc101cc477930
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2062389
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66311}