Introduce LookupIterator::PropertyOrElement which converts name to index if possible.

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index a28691503d12c7799ff3df9f0c99f130b211dd6b..482d7920d3dcce0007e29c8b6bec970fe37fb248 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -580,12 +580,9 @@ BasicJsonStringifier::Result BasicJsonStringifier::SerializeJSObject(
       } else {
         DCHECK(key->IsNumber());
         key_handle = factory()->NumberToString(Handle<Object>(key, isolate_));
-        uint32_t index;
         if (key->IsSmi()) {
           maybe_property = Object::GetElement(
               isolate_, object, Smi::cast(key)->value());
-        } else if (key_handle->AsArrayIndex(&index)) {
-          maybe_property = Object::GetElement(isolate_, object, index);
         } else {
           maybe_property = Object::GetPropertyOrElement(object, key_handle);
         }