[heap] Experiment: only compact on memory reducing GCs.

Bug: chromium:966832
Change-Id: If705f3b3a58f1fa316499fb7f645751fd1e6f8a3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1679507
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62415}

src/heap/mark-compact.cc

@@ -481,6 +481,9 @@ bool MarkCompactCollector::StartCompaction() {
   if (!compacting_) {
     DCHECK(evacuation_candidates_.empty());
 
+    if (FLAG_gc_experiment_less_compaction && !heap_->ShouldReduceMemory())
+      return false;
+
     CollectEvacuationCandidates(heap()->old_space());
 
     if (FLAG_compact_code_space) {