[runtime] Always request kMutable constness when reconfiguring to a field.

src/map-updater.cc

 // Copyright 2017 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/map-updater.h"
 
 #include "src/field-type.h"
 #include "src/handles.h"
 #include "src/isolate.h"
 #include "src/objects-inl.h"
@@ skipping 83 matching lines @@
   new_kind_ = kData;
   new_attributes_ = attributes;
   new_location_ = kField;
 
   PropertyDetails old_details =
       old_descriptors_->GetDetails(modified_descriptor_);
 
   // If property kind is not reconfigured merge the result with
   // representation/field type from the old descriptor.
   if (old_details.kind() == new_kind_) {
-    new_constness_ = old_details.constness();
+    new_constness_ = kMutable;
 
     Representation old_representation = old_details.representation();
     new_representation_ = representation.generalize(old_representation);
 
     Handle<FieldType> old_field_type =
         GetOrComputeFieldType(old_descriptors_, modified_descriptor_,
                               old_details.location(), new_representation_);
 
     new_field_type_ =
         Map::GeneralizeFieldType(old_representation, old_field_type,
@@ skipping 515 matching lines @@
   // the new descriptors to maintain descriptors sharing invariant.
   split_map->ReplaceDescriptors(*new_descriptors, *new_layout_descriptor);
 
   result_map_ = new_map;
   state_ = kEnd;
   return state_;  // Done.
 }
 
 }  // namespace internal
 }  // namespace v8