Let StressConcurrentAllocatorTask allocate small, medium and large
objects to test different code paths.

Bug: v8:10315
Change-Id: Ifff7e91bc95f0d926a58321b481183e9acf8bd32
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2335182
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69217}