Replace ad-hoc weakness in prototype transitions with WeakCell.

src/heap/mark-compact.cc

 // 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.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 2355 matching lines @@
 
 void MarkCompactCollector::ClearNonLivePrototypeTransitions(Map* map) {
   FixedArray* prototype_transitions =
       TransitionArray::GetPrototypeTransitions(map);
   int number_of_transitions =
       TransitionArray::NumberOfPrototypeTransitions(prototype_transitions);
 
   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);
       }
       new_number_of_transitions++;
     }
   }
 
   if (new_number_of_transitions != number_of_transitions) {
     TransitionArray::SetNumberOfPrototypeTransitions(prototype_transitions,
                                                      new_number_of_transitions);
   }
 
@@ skipping 2283 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }  // namespace internal
 }  // namespace v8