CompactionSpaces are only used during GC, so there is no need to
lock pending_allocation_mutex_ for them. Locking for GC allocations
actually caused multiple regressions.

Bug: chromium:1214765
Change-Id: I6db4ed96deced41dc52f04b2917ec944b4ccc674
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928189Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74896}