Recheck flag in StressConcurrentAllocationObserver to allow tests
to not run the StressConcurrentAllocatorTask even though Isolate was
already initialized.

Bug: v8:10315
Change-Id: Ia683458216821c103d1ae455330ebbd32d253905
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2403240Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69821}