style: Add 'overflow-anchor' CSS property.

https://drafts.csswg.org/css-scroll-anchoring/#exclusion-api

Differential Revision: https://phabricator.services.mozilla.com/D13265

components/style/properties/data.py

@@ -326,6 +326,7 @@ class Longhand(object):
                 "Opacity",
                 "OutlineStyle",
                 "Overflow",
+                "OverflowAnchor",
                 "OverflowClipBox",
                 "OverflowWrap",
                 "OverscrollBehavior",

components/style/properties/gecko.mako.rs

@@ -1419,6 +1419,7 @@ impl Clone for ${style_struct.gecko_struct_name} {
         "Number": impl_simple,
         "Opacity": impl_simple,
         "OverflowWrap": impl_simple,
+        "OverflowAnchor": impl_simple,
         "Perspective": impl_style_coord,
         "Position": impl_position,
         "RGBAColor": impl_rgba_color,

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

@@ -125,6 +125,18 @@ ${helpers.predefined_type(
     servo_restyle_damage = "reflow",
 )}
 
+${helpers.predefined_type(
+    "overflow-anchor",
+    "OverflowAnchor",
+    "computed::OverflowAnchor::Auto",
+    initial_specified_value="specified::OverflowAnchor::Auto",
+    products="gecko",
+    needs_context=False,
+    gecko_pref="layout.css.scroll-anchoring.enabled",
+    spec="https://drafts.csswg.org/css-scroll-anchoring/#exclusion-api",
+    animation_value_type="discrete",
+)}
+
 <% transition_extra_prefixes = "moz:layout.css.prefixes.transitions webkit" %>
 
 ${helpers.predefined_type(