Replace ad-hoc weakness in prototype transitions with WeakCell.

src/transitions.h

Index: src/transitions.h
diff --git a/src/transitions.h b/src/transitions.h
index 1cb91a222e6fec33133c3c3fd060102cb01a3750..b0aab9502e5d2e08dd810ae16aac23cc5edd7bf5 100644
--- a/src/transitions.h
+++ b/src/transitions.h
@@ -92,9 +92,8 @@ class TransitionArray: public FixedArray {
   //    0: finger - index of the first free cell in the cache
   //    1 + i: target map
   static const int kMaxCachedPrototypeTransitions = 256;
-  static Handle<Map> PutPrototypeTransition(Handle<Map> map,
-                                            Handle<Object> prototype,
-                                            Handle<Map> target_map);
+  static void PutPrototypeTransition(Handle<Map> map, Handle<Object> prototype,
+                                     Handle<Map> target_map);
 
   static Handle<Map> GetPrototypeTransition(Handle<Map> map,
                                             Handle<Object> prototype);