style: Fix cursor prefixed aliases to do the right thing.

This was an oversight in bug 1520154. We kept the -moz- version in the specified
value but not the computed value.

That's a very peculiar way of making aliases work. This makes them work
consistently as many other aliases instead.

Also, add an assert that would've caught this much much earlier.

Differential Revision: https://phabricator.services.mozilla.com/D40063

components/style/values/specified/ui.rs

@@ -199,7 +199,9 @@ pub enum CursorKind {
     Move,
     NoDrop,
     NotAllowed,
+    #[parse(aliases = "-moz-grab")]
     Grab,
+    #[parse(aliases = "-moz-grabbing")]
     Grabbing,
     EResize,
     NResize,
@@ -216,15 +218,9 @@ pub enum CursorKind {
     ColResize,
     RowResize,
     AllScroll,
+    #[parse(aliases = "-moz-zoom-in")]
     ZoomIn,
+    #[parse(aliases = "-moz-zoom-out")]
     ZoomOut,
     Auto,
-    #[cfg(feature = "gecko")]
-    MozGrab,
-    #[cfg(feature = "gecko")]
-    MozGrabbing,
-    #[cfg(feature = "gecko")]
-    MozZoomIn,
-    #[cfg(feature = "gecko")]
-    MozZoomOut,
 }