Currently, the serializer and deserializer assume that all top-level
declarations to be serialized will be objects.

This CL removes this assumption.

Bug: v8:11525, v8:11706
Change-Id: I5acf5e7a3b73aba5ffc5b1d5eb9cb51b3804a4af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2945178Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Vicky Kontoura <vkont@google.com>
Cr-Commit-Position: refs/heads/master@{#75020}