Reland "Replace ad-hoc weakness in prototype transitions with WeakCell."

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 53bd132ac0be4da9bc3241c4800e9ccc65dcd3d6..5a4a1a7036739a6aa79154c924a182b72a7442f0 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2404,11 +2404,13 @@ void MarkCompactCollector::ClearNonLivePrototypeTransitions(Map* map) {
   const int header = TransitionArray::kProtoTransitionHeaderSize;
   int new_number_of_transitions = 0;
   for (int i = 0; i < number_of_transitions; i++) {
-    Object* cached_map = prototype_transitions->get(header + i);
-    if (IsMarked(cached_map)) {
+    Object* cell = prototype_transitions->get(header + i);
+    if (!WeakCell::cast(cell)->cleared()) {
       if (new_number_of_transitions != i) {
-        prototype_transitions->set(header + new_number_of_transitions,
-                                   cached_map, SKIP_WRITE_BARRIER);
+        prototype_transitions->set(header + new_number_of_transitions, cell);
+        Object** slot = prototype_transitions->RawFieldOfElementAt(
+            header + new_number_of_transitions);
+        RecordSlot(slot, slot, cell);
       }
       new_number_of_transitions++;
     }