Introduce instance type for transition arrays.

src/transitions.cc

Index: src/transitions.cc
diff --git a/src/transitions.cc b/src/transitions.cc
index 85e5dc7967dccc65235a6b6e274489498eed2e2e..4fe2ace3d94f28736bcef8e94db59e208fc3fee3 100644
--- a/src/transitions.cc
+++ b/src/transitions.cc
@@ -391,18 +391,23 @@ int TransitionArray::Capacity(Object* raw_transitions) {
 Handle<TransitionArray> TransitionArray::Allocate(Isolate* isolate,
                                                   int number_of_transitions,
                                                   int slack) {
-  Handle<FixedArray> array = isolate->factory()->NewFixedArray(
-      LengthFor(number_of_transitions + slack), TENURED);
+  Handle<FixedArray> array = isolate->factory()->NewTransitionArray(
+      LengthFor(number_of_transitions + slack));
+  array->set(kNextLinkIndex, isolate->heap()->undefined_value());
   array->set(kPrototypeTransitionsIndex, Smi::FromInt(0));
   array->set(kTransitionLengthIndex, Smi::FromInt(number_of_transitions));
   return Handle<TransitionArray>::cast(array);
 }
 
 
-static void ZapTransitionArray(TransitionArray* transitions) {
-  MemsetPointer(transitions->data_start(),
+// static
+void TransitionArray::ZapTransitionArray(TransitionArray* transitions) {
+  // Do not zap the next link that is used by GC.
+  STATIC_ASSERT(kNextLinkIndex + 1 == kPrototypeTransitionsIndex);
+  MemsetPointer(transitions->data_start() + kPrototypeTransitionsIndex,
                 transitions->GetHeap()->the_hole_value(),
-                transitions->length());
+                transitions->length() - kPrototypeTransitionsIndex);
+  transitions->SetNumberOfTransitions(0);
 }