[heap] Also mark map when iterating black objects.

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 ec7594159ba63476e2cb69db8fb4876991eb4506..0c0f5b408ee5ae573dcdc8f533e7ad9d3987f620 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -310,7 +310,9 @@ void IncrementalMarking::IterateBlackObject(HeapObject* object) {
       // IterateBlackObject requires us to visit the whole object.
       page->ResetProgressBar();
     }
-    IncrementalMarkingMarkingVisitor::IterateBody(object->map(), object);
+    Map* map = object->map();
+    MarkGrey(heap_, map);
+    IncrementalMarkingMarkingVisitor::IterateBody(map, object);
   }
 }