Handle the case when an annotation is returned from the backend whose URL does
not match the frame URL and where there is no "main" guest frame. This can
happen in VitalSource, where no guest is created in the host frame.

The general solution to this is to change the client and h so that it can always
determine which query URL an annotation from h matched. See [1]. This is an
interim step which will work in VitalSource and similar scenarios.

[1] https://github.com/hypothesis/client/issues/4191