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/msg/constellation_msg.rs

@@ -541,7 +541,6 @@ pub enum LayoutHangAnnotation {
     UpdateScrollStateFromScript,
     RegisterPaint,
     SetNavigationStart,
-    GetRunningAnimations,
 }
 
 #[derive(Clone, Copy, Debug, Deserialize, Serialize)]