Synchronize on concurrent store buffer entries.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index d6ff1c96144232fc38bdeefa4d91001da58a6f7f..ffb54bb462cbd13dbb595d27b902a4c51eb38067 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -707,6 +707,9 @@ class MarkCompactCollector {
   // True if concurrent or parallel sweeping is currently in progress.
   bool sweeping_in_progress_;
 
+  // True if parallel compaction is currently in progress.
+  bool parallel_compaction_in_progress_;
+
   // Synchronize sweeper threads.
   base::Semaphore pending_sweeper_jobs_semaphore_;
 
@@ -873,6 +876,8 @@ class MarkCompactCollector {
 
   void EvacuatePagesInParallel();
 
+  void WaitUntilCompactionCompleted();
+
   void EvacuateNewSpaceAndCandidates();
 
   void ReleaseEvacuationCandidates();