Move most animation processing to script

This is preparation for sharing this code with layout_2020 and
implementing selective off-the-main-thread animations.

We still look for nodes not in the flow tree in the layout thread.

components/script/animation_timeline.rs

@@ -12,7 +12,7 @@ use time;
 /// A `AnimationTimeline` which is used to synchronize animations during the script
 /// event loop.
 #[derive(Clone, Copy, Debug, JSTraceable, MallocSizeOf)]
-pub struct AnimationTimeline {
+pub(crate) struct AnimationTimeline {
     current_value: f64,
 }