Implement animationiteration event

This event is triggered when an animation iterates. This change also
moves iteration out of style calculation to an "update animations" which
is the next part of having animation event handling match the HTML spec.

components/script/animations.rs

@@ -35,6 +35,30 @@ impl Animations {
         }
     }
 
+    pub(crate) fn update_for_new_timeline_value(
+        &mut self,
+        window: &Window,
+        now: f64,
+    ) -> AnimationsUpdate {
+        let mut update = AnimationsUpdate::new(window.pipeline_id());
+        let mut sets = self.sets.write();
+
+        for set in sets.values_mut() {
+            // When necessary, iterate our running animations to the next iteration.
+            for animation in set.animations.iter_mut() {
+                if animation.iterate_if_necessary(now) {
+                    update.add_event(
+                        animation.node,
+                        animation.name.to_string(),
+                        TransitionOrAnimationEventType::AnimationIteration,
+                        animation.active_duration(),
+                    );
+                }
+            }
+        }
+        update
+    }
+
     /// Processes any new animations that were discovered after reflow. Collect messages
     /// that trigger events for any animations that changed state.
     /// TODO(mrobinson): The specification dictates that this should happen before reflow.
@@ -255,6 +279,9 @@ pub enum TransitionOrAnimationEventType {
     TransitionCancel,
     /// "The animationend event occurs when the animation finishes"
     AnimationEnd,
+    /// "The animationiteration event occurs at the end of each iteration of an
+    /// animation, except when an animationend event would fire at the same time."
+    AnimationIteration,
 }
 
 impl TransitionOrAnimationEventType {
@@ -263,7 +290,7 @@ impl TransitionOrAnimationEventType {
     pub fn finalizes_transition_or_animation(&self) -> bool {
         match *self {
             Self::TransitionEnd | Self::TransitionCancel | Self::AnimationEnd => true,
-            Self::TransitionRun => false,
+            Self::TransitionRun | Self::AnimationIteration => false,
         }
     }
 
@@ -271,7 +298,7 @@ impl TransitionOrAnimationEventType {
     pub fn is_transition_event(&self) -> bool {
         match *self {
             Self::TransitionRun | Self::TransitionEnd | Self::TransitionCancel => true,
-            Self::AnimationEnd => false,
+            Self::AnimationEnd | Self::AnimationIteration => false,
         }
     }
 }