While LocalHeap allow to dereference handles on background threads, this
is only possible when FLAG_local_heaps is enabled.

Bug: v8:10315
Change-Id: Ia27582cda7e4b4faf4d2c76959de418415e1ae68
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2372147
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69557}