The preview field is used to display a glimpse of an object.
If the user wants to see more and expands the object, we make a request to the server to retrieve the properties of the object.

That means we shouldn't hold actors for the preview items.
This also means the the preview property is sometimes abused, and used as the placeholder for properties (e.g. for mapEntry items devtools/server/actors/object/property-iterator.js#305-309).

We should try to clean this up to ensure we don't consume unnecessary memory.