[heap] Add atomics to mark bit operations.

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index ce925491b4dc3b6dd3c5ead643c73baf75233132..3477261721b29ab74f192d10dc1a95ecb41631c1 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -141,7 +141,7 @@ void IncrementalMarking::TransferMark(Heap* heap, HeapObject* from,
   if (Marking::IsBlack(old_mark_bit)) {
     Marking::MarkBlack(new_mark_bit);
   } else if (Marking::IsGrey(old_mark_bit)) {
-    Marking::MarkGrey(new_mark_bit);
+    Marking::WhiteToGrey(new_mark_bit);
     heap->mark_compact_collector()->marking_deque()->Push(to);
     heap->incremental_marking()->RestartIfNotMarking();
   }