Replacing EnsureSweepingCompleted() with DrainSweepingWorklistForSpace()
caused sweeping to be completed later. This resulted in longer scavenger
pauses, since the scavenger had to stop & resume sweeping more often.
To avoid this, the scavenger now completes sweeping when all pages are
swept.

Bug: chromium:1106295, v8:10315
Change-Id: Ia46efdfb1f2492e9228f0349936be32a467f502b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2301935Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68900}