Update CSS animations in a SequentialTask.

We create the SequentialTask only if:

* We have no old computed values and we have animation name style in the new
  computed values.
* Any animation properties is changed.
* display property is changed from 'none' and we have animation name style.
* display property is changed to 'none'.

In a subsequent patch we skip the SequentialTask if we have no running
animations and the display propery is changed to 'none'.

components/script/layout_wrapper.rs

@@ -446,6 +446,10 @@ impl<'le> TElement for ServoLayoutElement<'le> {
     fn has_selector_flags(&self, flags: ElementSelectorFlags) -> bool {
         self.element.has_selector_flags(flags)
     }
+
+    fn update_animations(&self, _pseudo: Option<&PseudoElement>) {
+        panic!("this should be only called on gecko");
+    }
 }
 
 impl<'le> PartialEq for ServoLayoutElement<'le> {