Add support for animationend event

This is triggered when an animation finishes. This is a high priority
because it allows us to start rooting nodes with animations in the
script thread.

This doesn't yet cause a lot of tests to pass because they rely on the
existence of `Document.getAnimations()` and the presence of
`animationstart` and animationiteration` events.

components/style/animation.rs

@@ -172,6 +172,17 @@ impl KeyframesAnimationState {
         self.current_direction = old_direction;
         self.started_at = new_started_at;
     }
+
+    /// Calculate the active-duration of this animation according to
+    /// https://drafts.csswg.org/css-animations/#active-duration. active-duration
+    /// is not really meaningful for infinite animations so we just return 0
+    /// here in that case.
+    pub fn active_duration(&self) -> f64 {
+        match self.iteration_state {
+            KeyframesIterationState::Finite(_, max) => self.duration * (max as f64),
+            KeyframesIterationState::Infinite => 0.,
+        }
+    }
 }
 
 impl fmt::Debug for KeyframesAnimationState {