Always mark the entire valid prefix of the descriptor array.

src/objects-visiting-inl.h

Index: src/objects-visiting-inl.h
diff --git a/src/objects-visiting-inl.h b/src/objects-visiting-inl.h
index beb07b564497d7d87e044adbce43d278a6a12356..aaa8a613791d86c22a212d7ffb662fa977d99fc7 100644
--- a/src/objects-visiting-inl.h
+++ b/src/objects-visiting-inl.h
@@ -410,13 +410,6 @@ void StaticMarkingVisitor<StaticVisitor>::MarkMapContents(
   }
   int start = 0;
   int end = map->NumberOfOwnDescriptors();
-  Object* back_pointer = map->GetBackPointer();
-  if (!back_pointer->IsUndefined()) {
-    Map* parent_map = Map::cast(back_pointer);
-    if (descriptors == parent_map->instance_descriptors()) {
-      start = parent_map->NumberOfOwnDescriptors();
-    }
-  }
   if (start < end) {
     StaticVisitor::VisitPointers(heap,
         descriptors->GetDescriptorStartSlot(start),