Non HTML elements should be able to contribute to container timing

[Bas-moz]

We probably were a little enthusiastic in #63

64f5e12 made the lastPaintedElement an HTML element as well. This I believe is overzealous, I do think non-HTML elements can contribute to painting and should be reported.

I.e. I think in https://wicg.github.io/container-timing/#contributes-to-container-timing we should revert HTMLElement to Element, as well as in the WebIDL.

[Bas-moz]

(And the ContainerTimingRecord https://wicg.github.io/container-timing/#container-timing-record and other places as well)