Update interfaces/web-animations.idl (#19130)

Source: https://github.com/tidoust/reffy-reports/blob/97b8ae1/ed/idl/web-animations.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/586394436
diff --git a/interfaces/web-animations.idl b/interfaces/web-animations.idl
index 82681e5..8999389 100644
--- a/interfaces/web-animations.idl
+++ b/interfaces/web-animations.idl
@@ -23,7 +23,7 @@
 interface Animation : EventTarget {
              attribute DOMString                id;
              attribute AnimationEffect?         effect;
-             attribute AnimationTimeline?       timeline;
+    readonly attribute AnimationTimeline?       timeline;
              attribute double?                  startTime;
              attribute double?                  currentTime;
              attribute double                   playbackRate;