Free up 11 bits in fast-mode PropertyDetails by removing the enumeration-index.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index faa498a1bea89bf6a9d10186d6b030dae2b30ba7..719f41cb3116d9fa65af077b82fb88e1182cc93d 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -2316,8 +2316,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_FunctionSetReadOnlyPrototype) {
 
     CallbacksDescriptor new_desc(name,
         instance_desc->GetValue(index),
-        static_cast<PropertyAttributes>(details.attributes() | READ_ONLY),
-        details.descriptor_index());
+        static_cast<PropertyAttributes>(details.attributes() | READ_ONLY));
 
     // Create a new map featuring the new field descriptors array.
     Map* new_map;
@@ -2335,7 +2334,6 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_FunctionSetReadOnlyPrototype) {
     PropertyDetails new_details(
         static_cast<PropertyAttributes>(details.attributes() | READ_ONLY),
         details.type(),
-        Representation::None(),
         details.dictionary_index());
     function->property_dictionary()->DetailsAtPut(entry, new_details);
   }