Before this CL Oilpan would schedule concurrent marking and sweeping
even in the presence of --single-threaded-gc. This e.g. flakily breaks
Blink tests that do not set up a thread-pool in certain configurations
where they pass --single-threaded (implying --single-threaded-gc).

Bug: chromium:1300492
Change-Id: I64f0c6a20f9c29d689a62e63cc5a8d024962ff2c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3497760Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79321}