[resize-observer-1] Include SVG elements that generate CSS layout boxes #4032

[w1ndy]

This PR incorporates this resolution on the interactions between SVG and ResizeObserver into the spec:

RESOLVED: svg elements generating css layout boxes are included as part of resize observer and resize observer rectangles with a definition of `svg:root, :not(svg|) > svg, svg|foreignObject > svg { /* SVG elements with CSS layout box */}

I wonder if this resolution is still intended to be included in the spec. This PR can be closed if not. Thanks!

[w1ndy]

Created this PR because the last one (#4150) was closed due to inactivity.
Inviting spec owners @atotic @gregwhitworth , wondering if you have time to review this change?

[svgeesus]

This seems to me to correctly incorporate the resolution.

Approving but not merging, as the editors should look at it too.

[gregwhitworth]

Wow, thank you so much for landing this - I could have sworn I landed the other one; my bad. And thank you @emilio and @svgeesus for the review

[emilio]

@gregwhitworth Thanks for merging!

By the way, in the discussion in #7539 we guessed that you (and @atotic) were no longer interested in editing the ResizeObserver spec, but if by any chance you are please go ahead and re-add yourself as an editor, the more the merrier :)