style: Back out bug 1481866.

The behavior the WG proposed is way more subtle than what that bug implements,
including:

 * Implementing two logical overflow longhands.
 * Expanding the overflow shorthand to different longhands depending on the
   syntax of that.

Meanwhile, Blink hasn't done the swap and will ship the same behavior that we
shipped in Firefox 61 (bug 1453148), that is, overflow-x, then overflow-y.

So I think lacking a clear way forward we should revert this change and preserve
our shipped behavior.

Differential Revision: https://phabricator.services.mozilla.com/D6317

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

@@ -7,7 +7,7 @@
 <%helpers:shorthand
     name="overflow"
     flags="SHORTHAND_IN_GETCS"
-    sub_properties="overflow-y overflow-x"
+    sub_properties="overflow-x overflow-y"
     spec="https://drafts.csswg.org/css-overflow/#propdef-overflow"
 >
     use properties::longhands::overflow_x::parse as parse_overflow;
@@ -52,9 +52,9 @@
                 }
             }
         % endif
-        let overflow_y = parse_overflow(context, input)?;
-        let overflow_x =
-            input.try(|i| parse_overflow(context, i)).unwrap_or(overflow_y);
+        let overflow_x = parse_overflow(context, input)?;
+        let overflow_y =
+            input.try(|i| parse_overflow(context, i)).unwrap_or(overflow_x);
         Ok(expanded! {
             overflow_x: overflow_x,
             overflow_y: overflow_y,
@@ -63,10 +63,10 @@
 
     impl<'a> ToCss for LonghandsToSerialize<'a>  {
         fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
-            self.overflow_y.to_css(dest)?;
+            self.overflow_x.to_css(dest)?;
             if self.overflow_x != self.overflow_y {
                 dest.write_char(' ')?;
-                self.overflow_x.to_css(dest)?;
+                self.overflow_y.to_css(dest)?;
             }
             Ok(())
         }