Bug 1637398 [wpt PR 23547] - Update interfaces/web-animations.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/web-animations.idl (#23547)

Source: https://github.com/tidoust/reffy-reports/blob/d1fd24c/ed/idl/web-animations.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/686245019
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wpt-commits: 14bbee9665ad815a538863fc54b3a1a2b937b3d5
wpt-pr: 23547

testing/web-platform/tests/interfaces/web-animations.idl

@@ -139,6 +139,7 @@ interface mixin Animatable {
 
 dictionary KeyframeAnimationOptions : KeyframeEffectOptions {
     DOMString id = "";
+    AnimationTimeline? timeline;
 };
 
 dictionary GetAnimationsOptions {