[css-ui] border-image with an SVG resource that has no intrinsic size

I'm not sure if this is a UI spec thing, or borders, or something that's just fallen through the cracks. WebKit recently landed a change to make border-image with an SVG resource, that has no intrinsic size, behave like Firefox.

https://bugs.webkit.org/show_bug.cgi?id=139405 <https://bugs.webkit.org/show_bug.cgi?id=139405>

Example/Test is here: http://trac.webkit.org/export/184972/trunk/LayoutTests/fast/borders/border-image-fill-no-intrinsic-size.html

Chrome had the same behaviour as WebKit did, before the change. I haven't tested on IE/Edge (sorry... I really need to set up a login).

It's not completely clear what the best thing to do in this case is, but we think the Firefox behaviour was better. Is this specified anywhere? We'd really like to encourage people to not put an intrinsic size in their SVG.

Dean