Move field index into property details, freeing up the value slot of fields.

src/handles.cc

Index: src/handles.cc
diff --git a/src/handles.cc b/src/handles.cc
index 7a8d5c90bc5fa97ac289a600ebe0cfc68b243906..8b3a2db9d315f54dd55ce5681fd7be830062bfcf 100644
--- a/src/handles.cc
+++ b/src/handles.cc
@@ -802,7 +802,7 @@ Handle<FixedArray> GetEnumPropertyKeys(Handle<JSObject> object,
           if (details.type() != FIELD) {
             indices = Handle<FixedArray>();
           } else {
-            int field_index = Descriptor::IndexFromValue(descs->GetValue(i));
+            int field_index = descs->GetFieldIndex(i);
             if (field_index >= map->inobject_properties()) {
               field_index = -(field_index - map->inobject_properties() + 1);
             }