style: Let aspect-ratio (css-sizing-4) support 'auto | <ratio>'.

In order to test its parsing and serialization, we expose it but protect
it behind a pref.

Besides, I would like to drop layout.css.aspect-ratio-number.enabled in
the next patch because the spec has been updated. It seems we don't have
to keep this pref and we should always use Number.

Differential Revision: https://phabricator.services.mozilla.com/D74955

components/style/properties/data.py

@@ -328,6 +328,7 @@ class Longhand(object):
                 "AlignItems",
                 "AlignSelf",
                 "Appearance",
+                "AspectRatio",
                 "BreakBetween",
                 "BreakWithin",
                 "BackgroundRepeat",
@@ -364,7 +365,6 @@ class Longhand(object):
                 "MozScriptSizeMultiplier",
                 "TextDecorationSkipInk",
                 "NonNegativeNumber",
-                "Number",
                 "OffsetRotate",
                 "Opacity",
                 "OutlineStyle",