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}

src/heap/mark-compact.cc

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