Replace ad-hoc weakness in prototype transitions with WeakCell.

src/heap/objects-visiting-inl.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_OBJECTS_VISITING_INL_H_
 #define V8_OBJECTS_VISITING_INL_H_
 
 #include "src/heap/objects-visiting.h"
 
 namespace v8 {
@@ skipping 583 matching lines @@
       HeapObject::RawField(map, Map::kPointerFieldsEndOffset));
 }
 
 
 template <typename StaticVisitor>
 void StaticMarkingVisitor<StaticVisitor>::MarkTransitionArray(
     Heap* heap, TransitionArray* transitions) {
   if (!StaticVisitor::MarkObjectWithoutPush(heap, transitions)) return;
 
   if (transitions->HasPrototypeTransitions()) {
-    // Mark prototype transitions array but do not push it onto marking
-    // stack, this will make references from it weak. We will clean dead
-    // prototype transitions in ClearNonLiveReferences.
-    Object** slot = transitions->GetPrototypeTransitionsSlot();
-    HeapObject* obj = HeapObject::cast(*slot);
-    heap->mark_compact_collector()->RecordSlot(slot, slot, obj);
-    StaticVisitor::MarkObjectWithoutPush(heap, obj);
+    StaticVisitor::VisitPointer(heap,
+                                transitions->GetPrototypeTransitionsSlot());
   }
 
   int num_transitions = TransitionArray::NumberOfTransitions(transitions);
   for (int i = 0; i < num_transitions; ++i) {
     StaticVisitor::VisitPointer(heap, transitions->GetKeySlot(i));
   }
 }
 
 
 template <typename StaticVisitor>
@@ skipping 248 matching lines @@
 
   RelocIterator it(this, mode_mask);
   for (; !it.done(); it.next()) {
     it.rinfo()->template Visit<StaticVisitor>(heap);
   }
 }
 }
 }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_VISITING_INL_H_