[heap] Fix ProcessEphemeronsUntilFixpoint

This removes redundant worklist draining in the fix-point iteration. It
may discover more ephemerons and break the assertion.

The draining is not needed because FinishConcurrentMarking ensures
that all background threads drain their worklists.

Bug: chromium:1136405
Tbr: etiennep@chromium.org
Change-Id: I3a311dca36dd2f88ab5006f995be235c74c1a0f6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2464933Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70439}

src/heap/mark-compact.cc

@@ -1645,7 +1645,6 @@ void MarkCompactCollector::ProcessEphemeronsUntilFixpoint() {
       work_to_do = ProcessEphemerons();
       FinishConcurrentMarking(
           ConcurrentMarking::StopRequest::COMPLETE_ONGOING_TASKS);
-      DrainMarkingWorklist();
     }
 
     CHECK(weak_objects_.current_ephemerons.IsEmpty());