Intent to ship: new syntax of rgb(), rgba(), hsl(), and hsla() functions in CSS

[Xidorn Quan]

Summary:
The change we are shipping includes:
* making rgb() and hsl() accept an optional alpha value
* making rgb() and hsl() accept new whitespace-separated syntax e.g.
rgb(0 0 0 / 100%)
* making rgba() and hsla() alias of rgb() and hsl() correspondingly
(Please correct me if I was wrong, since I'm neither the implementor nor
the reviewer.)

Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1295456

Link to standard:
https://drafts.csswg.org/css-color-4/#rgb-functions
https://drafts.csswg.org/css-color-4/#the-hsl-notation

Platform coverage: all platforms

Estimated or target release:
Firefox 52 (The patch has been landed long ago without an Intent to
ship)

Preference behind which this will be implemented: No preference.

DevTools bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1302787 (also landed long
ago)

Do other browser engines implement this?
No any other browser engine is shipping this.

Tests
Several tests have been submitted to CSSWG:
https://github.com/w3c/csswg-test/tree/master/vendor-imports/mozilla/mozilla-central-reftests/color4

Note: It is a belated intent-to-ship, because people forgot to send it
when shipping this.