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

src/map-updater.cc

Index: src/map-updater.cc
diff --git a/src/map-updater.cc b/src/map-updater.cc
index e6dec658fd56ef531f7014a15cda175d42b0bff8..2f0d61a17c60dbf2efbb61efff23e0afc94d35ee 100644
--- a/src/map-updater.cc
+++ b/src/map-updater.cc
@@ -101,7 +101,7 @@ Handle<Map> MapUpdater::ReconfigureToDataField(int 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);