Cache animation computed values when animations change

Instead of recalculating the animation style every tick of an animation,
cache the computed values when animations change. In addition to being
more efficient, this will allow us to return animation rules as property
declarations because we don't need to consult the final style to produce
them.

components/layout_thread_2020/dom_wrapper.rs

@@ -592,8 +592,13 @@ impl<'le> TElement for ServoLayoutElement<'le> {
         false
     }
 
-    fn has_css_animations(&self) -> bool {
-        unreachable!("this should be only called on gecko");
+    fn has_css_animations(&self, context: &SharedStyleContext) -> bool {
+        context
+            .animation_states
+            .read()
+            .get(&self.as_node().opaque())
+            .map(|set| set.has_active_animation())
+            .unwrap_or(false)
     }
 
     fn has_css_transitions(&self) -> bool {

components/layout_thread_2020/lib.rs

@@ -574,7 +574,13 @@ impl LayoutThread {
         origin: ImmutableOrigin,
         animation_timeline_value: f64,
         animation_states: ServoArc<RwLock<FxHashMap<OpaqueNode, ElementAnimationSet>>>,
+        stylesheets_changed: bool,
     ) -> LayoutContext<'a> {
+        let traversal_flags = match stylesheets_changed {
+            true => TraversalFlags::ForCSSRuleChanges,
+            false => TraversalFlags::empty(),
+        };
+
         LayoutContext {
             id: self.id,
             origin,
@@ -586,7 +592,7 @@ impl LayoutThread {
                 animation_states,
                 registered_speculative_painters: &self.registered_painters,
                 current_time_for_animations: animation_timeline_value,
-                traversal_flags: TraversalFlags::empty(),
+                traversal_flags,
                 snapshot_map: snapshot_map,
             },
             image_cache: self.image_cache.clone(),
@@ -1064,6 +1070,7 @@ impl LayoutThread {
             origin,
             data.animation_timeline_value,
             data.animations.clone(),
+            data.stylesheets_changed,
         );
 
         let dirty_root = unsafe {