style: Not all computed value flags are really inherited.

This fixes the fishiness I noticed in:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1407832

components/style/properties/properties.mako.rs

@@ -2872,7 +2872,7 @@ impl<'a> StyleBuilder<'a> {
             /* rules = */ None,
             parent.custom_properties().cloned(),
             parent.writing_mode,
-            parent.flags,
+            parent.flags.inherited(),
             visited_style,
         )
     }