Remove the SWEEPING state from incremental marking. Sweeping is now
always completed when starting incremental marking. Before this change
there needed to be a safepoint each for starting marking and completing
sweeping. Now both happens within a single safepoint.

Bug: v8:10315
Change-Id: Iad2835554865f2de24376372affe9a98992d1fa0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190419Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67678}