Wait for concurrent unmapping tasks in GC prologue.

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 738e00b09ceb6fd0a4ae804dbe3b1e27aef125c0..590f5cb60c3d1ef9f70564918c450c27ea410f46 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -550,8 +550,6 @@ void MarkCompactCollector::EnsureSweepingCompleted() {
     pending_sweeper_jobs_semaphore_.Wait();
   }
 
-  heap()->WaitUntilUnmappingOfFreeChunksCompleted();
-
   ParallelSweepSpacesComplete();
   sweeping_in_progress_ = false;
   RefillFreeList(heap()->paged_space(OLD_SPACE));
@@ -812,6 +810,10 @@ void MarkCompactCollector::Prepare() {
     EnsureSweepingCompleted();
   }
 
+  // If concurrent unmapping tasks are still running, we should wait for
+  // them here.
+  heap()->WaitUntilUnmappingOfFreeChunksCompleted();
+
   // Clear marking bits if incremental marking is aborted.
   if (was_marked_incrementally_ && heap_->ShouldAbortIncrementalMarking()) {
     heap()->incremental_marking()->Stop();