PageTitle - PageURL in Title sounds good. If page URL is stored in a custom field, different PDF readers might not display that field anywhere. While Subject field is not used as often, it is a part of the PDF specification (Document Information Dictionary) and its use does not fall far from the tree if we say that the subject of the document is the URL from which PDF is saved from. 😀

As for the Chromium's PDF viewer UI in general, instead of just handling one specific field, it would be better to display all properties from PDF's Document Information Dictionary. This way it doesn't matter if the URL is in a custom or a standard field.