`<iframe>` and `<img>` tags share the same spec for several aspects of
lazy-loading: how the `loading` attribute works, the "will lazy load
element" steps, and a member for storing the lazy-load resumption
steps. So let's share the implementation by using a base class.
This mostly involves moving things around. However, we also change the
`start_intersection_observing_a_lazy_loading_element()` method to take
a LazyLoadingElement, and operate on one, instead of always casting to
HTMLImageElement.
We do unfortunately have to do some shenanigans to make the cast work,
by adding a virtual function stub in DOM::Element.
