Intent to prototype and ship: font-family: system-ui

[Emilio Cobos Álvarez]

Summary: Allow authors to use a system font in a font family list.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1226042

Standard: https://drafts.csswg.org/css-fonts/#system-font

Platform coverage: all

Preference: layout.css.system-ui.enabled

DevTools bug: The font panel allows authors to see the underlying font.

Other browsers: Both WebKit and Blink ship this since quite a while ago.

web-platform-tests: I added some tests for this in bug 1722488 (there
were none before) and Jonathan added some more in bug 1723552.

Security and Privacy: Note that this is not exposing more information
than before. Authors can access the same fonts via the existing system
fonts in the font shorthand. But system-ui in the font-family is a more
convenient way to access the system font family.

Cheers,

-- Emilio