style: Prototype :focus-visible behind a flag.

The heuristic is that we show focus outlines for unknown or key focus, and not
for mouse / touch.

This is probably not the final heuristic we take, but this allows people to play
with it and file bugs.

Once this is mature enough we should remove :-moz-focusring in favor of
:focus-visible.

Differential Revision: https://phabricator.services.mozilla.com/D63861

components/style/element_state.rs

@@ -137,6 +137,10 @@ bitflags! {
         const IN_AUTOFILL_STATE = 1 << 50;
         /// Non-standard & undocumented.
         const IN_AUTOFILL_PREVIEW_STATE = 1 << 51;
+        /// :focus-visible
+        ///
+        /// https://drafts.csswg.org/selectors-4/#the-focus-visible-pseudo
+        const IN_FOCUS_VISIBLE_STATE = 1 << 52;
     }
 }