This CL completes sweeping in Heap::PerformGarbageCollection before
invoking the actual collection. Collection code can now assume that
sweeping was already finished.

This helps with emitting the right epoch for sweeping and avoids a
data-race when updating the epoch while sweeping tasks are still running.

Bug: chromium:1154636
Change-Id: Ic9c4ac49568199d0ea48f17eea132079defe74a5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2573478
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71794}