Prevent deadlock on shutdown by allowing all allocations on background
threads after tear down was started. Background threads need the main
thread to perform the collection, which never happens when V8 is already
shutting down.

Bug: v8:10315
Change-Id: I5d3358229624aead7b0ebcfee7e5840315f23329
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2230537
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68199}