Move Maps' back pointers from "transitions" to "constructor" field

src/type-info.cc

Index: src/type-info.cc
diff --git a/src/type-info.cc b/src/type-info.cc
index 01943414a7cb60d8495bab94a0adacf13149dd4b..0e23341a445191a97d8f370a57dd4ade7d6e7fbe 100644
--- a/src/type-info.cc
+++ b/src/type-info.cc
@@ -422,7 +422,7 @@ bool TypeFeedbackOracle::CanRetainOtherContext(Map* map,
                                                Context* native_context) {
   Object* constructor = NULL;
   while (!map->prototype()->IsNull()) {
-    constructor = map->constructor();
+    constructor = map->GetConstructor();
     if (!constructor->IsNull()) {
       // If the constructor is not null or a JSFunction, we have to
       // conservatively assume that it may retain a native context.
@@ -435,7 +435,7 @@ bool TypeFeedbackOracle::CanRetainOtherContext(Map* map,
     }
     map = HeapObject::cast(map->prototype())->map();
   }
-  constructor = map->constructor();
+  constructor = map->GetConstructor();
   if (constructor->IsNull()) return false;
   JSFunction* function = JSFunction::cast(constructor);
   return CanRetainOtherContext(function, native_context);