Switch to the new scroll-snap-type syntax for the old scroll snap implementation and drop the scroll-snap-type-{x,y} longhands.

Now scroll-snap-type is a longhand property.

Depends on D21621

Differential Revision: https://phabricator.services.mozilla.com/D21622

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1312163
gecko-commit: 1703fb877b13222123f73c960d368e30b5a13188
gecko-integration-branch: central
gecko-reviewers: emilio
diff --git a/css/css-scroll-snap/inheritance.html b/css/css-scroll-snap/inheritance.html
index b380cb4..2569cf3 100644
--- a/css/css-scroll-snap/inheritance.html
+++ b/css/css-scroll-snap/inheritance.html
@@ -33,7 +33,7 @@
 assert_not_inherited('scroll-padding-top', 'auto', '10px');
 assert_not_inherited('scroll-snap-align', 'none', 'start end');
 assert_not_inherited('scroll-snap-stop', 'normal', 'always');
-assert_not_inherited('scroll-snap-type', 'none', 'inline proximity');
+assert_not_inherited('scroll-snap-type', 'none', 'inline');
 </script>
 </body>
 </html>
diff --git a/css/css-scroll-snap/parsing/scroll-snap-type-valid.html b/css/css-scroll-snap/parsing/scroll-snap-type-valid.html
index fde2211..59a0cb9 100644
--- a/css/css-scroll-snap/parsing/scroll-snap-type-valid.html
+++ b/css/css-scroll-snap/parsing/scroll-snap-type-valid.html
@@ -20,7 +20,7 @@
 test_valid_value("scroll-snap-type", "both");
 
 test_valid_value("scroll-snap-type", "y mandatory");
-test_valid_value("scroll-snap-type", "inline proximity");
+test_valid_value("scroll-snap-type", "inline proximity", "inline"); // The shortest serialization is preferable
 </script>
 </body>
 </html>
diff --git a/web-animations/animation-model/animation-types/property-list.js b/web-animations/animation-model/animation-types/property-list.js
index 8d9b296..3ac142e 100644
--- a/web-animations/animation-model/animation-types/property-list.js
+++ b/web-animations/animation-model/animation-types/property-list.js
@@ -1175,18 +1175,6 @@
       { type: 'discrete', options: [ [ 'auto', 'smooth' ] ] }
     ]
   },
-  'scroll-snap-type-x': {
-    // https://developer.mozilla.org/en/docs/Web/CSS/scroll-snap-type-x
-    types: [
-      { type: 'discrete', options: [ [ 'mandatory', 'proximity' ] ] }
-    ]
-  },
-  'scroll-snap-type-y': {
-    // https://developer.mozilla.org/en/docs/Web/CSS/scroll-snap-type-y
-    types: [
-      { type: 'discrete', options: [ [ 'mandatory', 'proximity' ] ] }
-    ]
-  },
   'shape-outside': {
     // http://dev.w3.org/csswg/css-shapes/#propdef-shape-outside
     types: [