style: Add support for parsing the content-visibility property from the CSS Contain specification

Add initial parsing support for the CSS `content-visibility` attribute.
Currently these parsed values have no effect.

Differential Revision: https://phabricator.services.mozilla.com/D140834

components/style/properties/counted_unknown_properties.py

@@ -120,5 +120,4 @@ COUNTED_UNKNOWN_PROPERTIES = [
     "-webkit-columns",
     "-webkit-column-rule-color",
     "-webkit-shape-margin",
-    "content-visibility",
 ]

components/style/properties/data.py

@@ -462,6 +462,7 @@ class Longhand(Property):
                 "Clear",
                 "ColumnCount",
                 "Contain",
+                "ContentVisibility",
                 "Display",
                 "FillRule",
                 "Float",

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

@@ -613,6 +613,16 @@ ${helpers.predefined_type(
     spec="https://drafts.csswg.org/css-contain/#contain-property",
 )}
 
+${helpers.predefined_type(
+    "content-visibility",
+    "ContentVisibility",
+    "computed::ContentVisibility::Visible",
+    engines="gecko",
+    spec="https://drafts.csswg.org/css-contain/#content-visibility",
+    gecko_pref="layout.css.content-visibility.enabled",
+    animation_value_type="none",
+)}
+
 ${helpers.predefined_type(
     "appearance",
     "Appearance",