Intent to ship: Infer aspect-ratio from img width and height attributes.

[Emilio Cobos Álvarez]

Hi,

As of today I intend to turn this feature on for release and beta
channels (it was enabled only on Nightly until now).

It's been developed behind the
layout.css.width-and-height-map-to-aspect-ratio.enabled pref.

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1585637

This was implemented in bug 1547231.

Spec link is
https://html.spec.whatwg.org/multipage/rendering.html#attributes-for-embedded-content-and-images
(spec PR: https://github.com/whatwg/html/pull/4952).

Platform coverage: All

web-platform-tests:
https://github.com/web-platform-tests/wpt/blob/abb820ae83ffb82ccd8380ce61685b51329a7b07/html/rendering/replaced-elements/attributes-for-embedded-content-and-images/img-aspect-ratio.tentative.html
(which I guess I should rename now that there's a spec).

Relevant browser bugs:

Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=979891
Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1547231
Safari: https://bugs.webkit.org/show_bug.cgi?id=201641

Secure contexts: not restricted to secure contexts, as many layout features.

Is this feature enabled by default in sandboxed iframes? Yes

Let me know if there's any concern with this.

Thanks,

-- Emilio