[heap] Fix waiting for parallel tasks

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 d51b03f64f1537bc41a0d20ad16d417ef78727ed..6d93216cd452520017dbbed1557915c3c7ee7b1b 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -3464,8 +3464,9 @@ void MarkCompactCollector::EvacuatePagesInParallel() {
 
 
 void MarkCompactCollector::WaitUntilCompactionCompleted() {
-  while (concurrent_compaction_tasks_active_-- > 0) {
+  while (concurrent_compaction_tasks_active_ > 0) {
     pending_compaction_tasks_semaphore_.Wait();
+    concurrent_compaction_tasks_active_--;
   }
   parallel_compaction_in_progress_ = false;
 }