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 a885c4ac03ca60172e8003ab0c662696da950026..12ab5102824b0606927d1828b9572657acee8323 100644
--- a/test/cctest/test-heap.cc
+++ b/test/cctest/test-heap.cc
@@ -2466,7 +2466,8 @@ 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.