[heap] Concurrently free empty slot set buckets.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 38c32befdb16f1b02f5097d5fadb14a836074793..4b8033d11789827bcbfcc19194be6fdb5ffb0e44 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -3830,6 +3830,9 @@ int MarkCompactCollector::Sweeper::ParallelSweepPage(Page* page,
     if (page->typed_old_to_new_slots()) {
       page->typed_old_to_new_slots()->FreeToBeFreedChunks();
     }
+    if (page->old_to_new_slots()) {
+      page->old_to_new_slots()->FreeToBeFreedBuckets();
+    }
 
     {
       base::LockGuard<base::Mutex> guard(&mutex_);