GetMaxConcurrency() uses queue size to ensure enough threads.

Change-Id: I1f091da91928c196813630fbabf54f7e68f87ead
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2480942
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70711}