ManualGCScope is used in tests that perform GC manually. Stressing
concurrent allocation interferes with that and may trigger more GCs
than the test expects.

Bug: v8:10315
Change-Id: I6705f0b7cc555074b319a41d29810936b5a2a556
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2392242Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69691}