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",

components/style/properties/longhands/position.mako.rs

@@ -447,17 +447,13 @@ ${helpers.predefined_type(
     servo_restyle_damage="reflow",
 )}
 
-// NOTE(emilio): Before exposing this property to content, we probably need to
-// change syntax and such, and make it apply to more elements.
-//
-// For now, it's used only for mapped attributes.
 ${helpers.predefined_type(
     "aspect-ratio",
-    "Number",
-    "computed::Number::zero()",
+    "AspectRatio",
+    "computed::AspectRatio::auto()",
     engines="gecko servo-2013",
-    animation_value_type="ComputedValue",
-    spec="Internal, for now",
-    enabled_in="",
+    animation_value_type="discrete",
+    spec="https://drafts.csswg.org/css-sizing-4/#aspect-ratio",
+    gecko_pref="layout.css.aspect-ratio.enabled",
     servo_restyle_damage="reflow",
 )}