style: Implement scroll-snap-align parser and serializer.

https://drafts.csswg.org/css-scroll-snap-1/#scroll-snap-align

Differential Revision: https://phabricator.services.mozilla.com/D20205

components/style/properties/data.py

@@ -339,6 +339,7 @@ class Longhand(object):
                 "Resize",
                 "SVGOpacity",
                 "SVGPaintOrder",
+                "ScrollSnapAlign",
                 "ScrollSnapType",
                 "TextAlign",
                 "TextDecorationLine",

components/style/properties/gecko.mako.rs

@@ -1354,6 +1354,7 @@ impl Clone for ${style_struct.gecko_struct_name} {
         "Appearance": impl_simple,
         "OverscrollBehavior": impl_simple,
         "OverflowClipBox": impl_simple,
+        "ScrollSnapAlign": impl_simple,
         "ScrollSnapType": impl_simple,
         "Float": impl_simple,
         "Overflow": impl_simple,

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

@@ -417,6 +417,16 @@ ${helpers.single_keyword(
     animation_value_type="discrete",
 )}
 
+${helpers.predefined_type(
+    "scroll-snap-align",
+    "ScrollSnapAlign",
+    "computed::ScrollSnapAlign::none()",
+    products="gecko",
+    gecko_pref="layout.css.scroll-snap-v1.enabled",
+    spec="https://drafts.csswg.org/css-scroll-snap-1/#scroll-snap-align",
+    animation_value_type="discrete",
+)}
+
 % for axis in ["x", "y"]:
     ${helpers.predefined_type(
         "scroll-snap-type-" + axis,