Replace ad-hoc weakness in prototype transitions with WeakCell.

test/cctest/test-heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2448 matching lines @@
   CcTest::heap()->CollectAllGarbage();
   const int transitions = 10 - 3;
   CHECK_EQ(initialTransitions + transitions,
            NumberOfProtoTransitions(baseObject->map()));
 
   // Verify that prototype transitions array was compacted.
   FixedArray* trans =
       TransitionArray::GetPrototypeTransitions(baseObject->map());
   for (int i = initialTransitions; i < initialTransitions + transitions; i++) {
     int j = TransitionArray::kProtoTransitionHeaderSize + i;
-    CHECK(trans->get(j)->IsMap());
+    CHECK(trans->get(j)->IsWeakCell());
+    CHECK(WeakCell::cast(trans->get(j))->value()->IsMap());
   }
 
   // Make sure next prototype is placed on an old-space evacuation candidate.
   Handle<JSObject> prototype;
   PagedSpace* space = CcTest::heap()->old_space();
   {
     AlwaysAllocateScope always_allocate(isolate);
     SimulateFullSpace(space);
     prototype =
         factory->NewJSArray(32 * KB, FAST_HOLEY_ELEMENTS, WEAK, TENURED);
@@ skipping 3418 matching lines @@
   size_t counter2 = 2000;
   tracer->SampleAllocation(time2, counter2, counter2);
   size_t throughput = tracer->AllocationThroughputInBytesPerMillisecond(100);
   CHECK_EQ(2 * (counter2 - counter1) / (time2 - time1), throughput);
   int time3 = 1000;
   size_t counter3 = 30000;
   tracer->SampleAllocation(time3, counter3, counter3);
   throughput = tracer->AllocationThroughputInBytesPerMillisecond(100);
   CHECK_EQ(2 * (counter3 - counter1) / (time3 - time1), throughput);
 }