Since main thread allocation does not start incremental marking anymore
while holding allocation_mutex_, background allocation does not need
ParkedMutexGuard anymore to avoid deadlocks.

This also means background thread allocation isn't paused anymore to
perform a GC, which already resulted in subtle bugs (e.g. in
ExpandBackground with incremental marking). We also do not
stop-the-world anymore while holding allocation_mutex_.

Bug: v8:10315
Change-Id: Iadf00bc26434c765722b82a10497ab06151f15cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2289771Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68754}