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/gecko/wrapper.rs

@@ -2064,6 +2064,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             NonTSPseudoClass::MozReadOnly |
             NonTSPseudoClass::MozReadWrite |
             NonTSPseudoClass::FocusWithin |
+            NonTSPseudoClass::FocusVisible |
             NonTSPseudoClass::MozDragOver |
             NonTSPseudoClass::MozDevtoolsHighlighted |
             NonTSPseudoClass::MozStyleeditorTransitioning |