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

test/cctest/test-heap.cc

Index: test/cctest/test-heap.cc
diff --git a/test/cctest/test-heap.cc b/test/cctest/test-heap.cc
index f0dc643fc60f3a06b65e190bdc534ff7aca9735b..0a4cf2b1e4006a7983ef6cd158a252d116ea94e3 100644
--- a/test/cctest/test-heap.cc
+++ b/test/cctest/test-heap.cc
@@ -2521,7 +2521,8 @@ TEST(PrototypeTransitionClearing) {
       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.