Replace ad-hoc weakness in transition array with WeakCell.

src/heap/objects-visiting.h

Index: src/heap/objects-visiting.h
diff --git a/src/heap/objects-visiting.h b/src/heap/objects-visiting.h
index 1b788e893b7b23d0236d4acbe5e59ef61bee4f23..9d1634516d89120bb84400ccba04134a098467ec 100644
--- a/src/heap/objects-visiting.h
+++ b/src/heap/objects-visiting.h
@@ -438,7 +438,6 @@ class StaticMarkingVisitor : public StaticVisitorBase {
   // Mark pointers in a Map and its TransitionArray together, possibly
   // treating transitions or back pointers weak.
   static void MarkMapContents(Heap* heap, Map* map);
-  static void MarkTransitionArray(Heap* heap, TransitionArray* transitions);
 
   // Code flushing support.
   INLINE(static bool IsFlushable(Heap* heap, JSFunction* function));