style: no-op visited changes earlier if visited links are disabled.

We force a repaint from ContentStateChangedInternal if visited links are
disabled, and that's observable. Let's cut it off as early as we can to avoid
timing attacks even when :visited is disabled.

Differential Revision: https://phabricator.services.mozilla.com/D3304

components/style/invalidation/element/state_and_attributes.rs

@@ -167,8 +167,13 @@ where
         // do for this case.
         if state_changes.intersects(ElementState::IN_VISITED_OR_UNVISITED_STATE) {
             trace!(" > visitedness change, force subtree restyle");
+            // If we get here with visited links disabled, we should probably
+            // just avoid the restyle and remove the state change here, not only
+            // as an optimization, but also because it kind of would kill the
+            // point of disabling visited links.
+            debug_assert!(self.shared_context.visited_styles_enabled);
             // We can't just return here because there may also be attribute
-            // changes as well that imply additional hints.
+            // changes as well that imply additional hints for siblings.
             self.data.hint.insert(RestyleHint::restyle_subtree());
         }