Revert "Preempt ConcurrentMarking tasks ASAP when cancelling marking."

This reverts commit 8b53b9d9.

Reason for revert: Several GC failures, e.g. https://build.chromium.org/p/client.v8/builders/V8%20Linux/builds/23236, https://build.chromium.org/p/client.v8/builders/V8%20Mac/builds/18390 

Original change's description:
> Preempt ConcurrentMarking tasks ASAP when cancelling marking.
> 
> Follow-up to https://chromium-review.googlesource.com/c/v8/v8/+/924867
> 
> Bug: chromium:812178
> Change-Id: I2abe28c6e953df42cffdcbd7ea35df9d29849905
> Reviewed-on: https://chromium-review.googlesource.com/924030
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51357}

TBR=gab@chromium.org,ulan@chromium.org,mlippautz@chromium.org

Change-Id: Ic4e226fdd02d8259244cef46e9923c95e6606cc4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:812178
Reviewed-on: https://chromium-review.googlesource.com/924425Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51362}

src/heap/mark-compact.cc

@@ -866,7 +866,9 @@ void MarkCompactCollector::Prepare() {
   if (was_marked_incrementally_ && heap_->ShouldAbortIncrementalMarking()) {
     heap()->incremental_marking()->Stop();
     heap()->incremental_marking()->AbortBlackAllocation();
-    FinishConcurrentMarking(ConcurrentMarking::StopRequest::PREEMPT_TASKS);
+    // TODO(gab): PREEMPT_TASKS here in a follow-up CL.
+    FinishConcurrentMarking(
+        ConcurrentMarking::StopRequest::COMPLETE_ONGOING_TASKS);
     heap()->incremental_marking()->Deactivate();
     ClearMarkbits();
     AbortWeakCollections();