Garbage collection requests from background threads are ignored if
the heap is tearing down. This fixes CanExpandOldGenerationBackground
to check for that case.

Bug: v8:10315
Change-Id: I79b6a4446bf3c9037dbca54849c87f022be76b49
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2387964
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69666}