Starting the task right on NotifyDeserializationComplete() turned out
to be too early for some tests. So let an AllocationObserver start
the stress allocation after deserialization is finished.

Bug: v8:10315
Change-Id: I00a4294add5ec87074f9e775c602b3031cc16d58
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2358735
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69441}