Re-land "Concurrently unmap free pages."

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 af6894c5755b2a6d7a14a853c3d3fd1ddb83de73..4c8db42035bb8c5722fe9b2fbe73b35d87f43f54 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -519,12 +519,15 @@ void MarkCompactCollector::EnsureSweepingCompleted() {
     SweepInParallel(heap()->paged_space(CODE_SPACE), 0);
     SweepInParallel(heap()->paged_space(MAP_SPACE), 0);
   }
-  // Wait twice for both jobs.
+
   if (heap()->concurrent_sweeping_enabled()) {
     pending_sweeper_jobs_semaphore_.Wait();
     pending_sweeper_jobs_semaphore_.Wait();
     pending_sweeper_jobs_semaphore_.Wait();
   }
+
+  heap()->WaitUntilUnmappingOfFreeChunksCompleted();
+
   ParallelSweepSpacesComplete();
   sweeping_in_progress_ = false;
   RefillFreeList(heap()->paged_space(OLD_SPACE));