This will enable some fuzzers to alter the thread-pool size.

Bug: v8:7455
Change-Id: Ic9c9600cdb3dc50e860dbda8432a23bb20f1dd44
Reviewed-on: https://chromium-review.googlesource.com/924273Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51335}