style: Cache computed styles objects display: none subtrees

This reuses our existing undisplayed style generation, but in a
per-document rather than per-nsComputedDOMStyle object, which means that
we can avoid re-resolving styles of elements in display: none subtrees
much more often.

This brings the test-case in the bug to par with other browsers or
better, and is much simpler than the initial approach I tried back in
the day.

Differential Revision: https://phabricator.services.mozilla.com/D147547

components/style/gecko/media_queries.rs

@@ -411,6 +411,11 @@ impl Device {
         self.used_font_metrics.load(Ordering::Relaxed)
     }
 
+    /// Returns whether visited styles are enabled.
+    pub fn visited_styles_enabled(&self) -> bool {
+        unsafe { bindings::Gecko_VisitedStylesEnabled(self.document()) }
+    }
+
     /// Returns the device pixel ratio.
     pub fn device_pixel_ratio(&self) -> Scale<f32, CSSPixel, DevicePixel> {
         let pc = match self.pres_context() {