Move mutable timelines to web-animations-2

Author: stephenmcgruer

web-animations-1/Overview.bs

@@ -3951,7 +3951,7 @@ API by the {{Animation}} interface.
 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;
@@ -4021,8 +4021,6 @@ interface Animation : EventTarget {
     the procedure to <a>set the target effect of an animation</a>.
 :   <dfn attribute for=Animation>timeline</dfn>
 ::  The <a>timeline</a> associated with this animation.
-    Setting this attribute updates the object's <a>timeline</a> using
-    the procedure to <a>set the timeline of an animation</a>.
 :   <dfn attribute for=Animation>startTime</dfn>
 ::  Returns the [=start time=] of this animation.
     Setting this attribute updates the [=start time=] using

web-animations-2/Overview.bs

@@ -70,6 +70,7 @@ urlPrefix: https://drafts.csswg.org/web-animations-1/; type: dfn; spec: web-anim
     text: playback direction
     text: pause an animation
     text: ready
+    text: set the timeline of an animation
     text: simple iteration progress
     text: target effect
     text: target element
@@ -1512,6 +1513,26 @@ partial interface AnimationTimeline {
 
 </div>
 
+<h3 id="the-animation-interface">The <code>Animation</code> interface</h3>
+
+The <a>Animation</a> interface is amended to make the <var>timeline</var> member mutable:
+
+<pre class='idl'>
+[Exposed=Window,
+ Constructor(optional AnimationEffect? effect = null,
+             optional AnimationTimeline? timeline)]
+interface Animation : EventTarget {
+               attribute AnimationTimeline?       timeline;
+};
+</pre>
+
+<div class='attributes'>
+
+:   <dfn attribute for=Animation>timeline</dfn>
+::  The <a>timeline</a> associated with this animation.
+    Setting this attribute updates the object's <a>timeline</a> using
+    the procedure to <a>set the timeline of an animation</a>.
+
 <h3 id="the-animationeffectreadonly-interface">The <code>AnimationEffectReadOnly</code> interface</h3>