The timer might not be started when the main thread starts shutdown
between a background thread invoking RequestGC() and
AwaitCollectionBackground().

Add early bailout to AwaitCollectionBackground() in case shutdown
was already initiated.

Bug: v8:11823
Change-Id: Id646cdefa99adb04553c21337ad19538071ee3d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2919957Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74808}