Intent to unship: 3-valued position syntax from <object-position>, <perspective-origin>, <mask-position>

[Boris Chiou]

Hi,

The spec for <position>
<https://drafts.csswg.org/css-values-4/#typedef-position> only accepts 1,2,
or 4-valued syntax. so I'd like to retire the 3-valued positions from these
CSS properties (i.e. <object-position>, <perspective-origin>, and
<mask-position>). Blink retired this support in Chrome 68, and we are
planning to retire them to match the spec, too.

(Note: <background-position> still accepts 3-valued syntax, per the
definition of <bg-position>
<https://drafts.csswg.org/css-backgrounds-3/#typedef-bg-position>.)

Spec change: https://github.com/w3c/csswg-drafts/issues/2140
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1559276