cppgc: Bugfix for marking not-fully-constructed objects

Bug: chromium:1056170
Change-Id: I92b36c8ac3d31837729f2e1ce9f3c756a6270da1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2248182
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68367}

src/heap/cppgc/marker.cc

@@ -218,7 +218,7 @@ void Marker::MarkNotFullyConstructedObjects() {
   NotFullyConstructedWorklist::View view(&not_fully_constructed_worklist_,
                                          kMutatorThreadId);
   while (view.Pop(&item)) {
-    heap().stack()->IteratePointers(marking_visitor_.get());
+    marking_visitor_->TraceConservativelyIfNeeded(item);
   }
 }