Introduce --stress-concurrent-allocation flag. With this flag, V8 will
run periodically start an allocation background task, which allocates
objects. Will be used for testing background allocation.

Bug: v8:10315
Change-Id: Iddb7ff34601a492bc2f26f41cc56a2a899807cfa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2228889Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68172}