Update interfaces/web-animations.idl (#23723)

Source: https://github.com/tidoust/reffy-reports/blob/4a0d07a/ed/idl/web-animations.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/689477816
diff --git a/interfaces/web-animations.idl b/interfaces/web-animations.idl
index 64265c3..ae64bb8 100644
--- a/interfaces/web-animations.idl
+++ b/interfaces/web-animations.idl
@@ -6,6 +6,7 @@
 [Exposed=Window]
 interface AnimationTimeline {
     readonly attribute double? currentTime;
+    readonly attribute TimelinePhase phase;
 };
 
 dictionary DocumentTimelineOptions {
@@ -49,6 +50,8 @@
 
 enum AnimationReplaceState { "active", "removed", "persisted" };
 
+enum TimelinePhase { "inactive", "before", "active", "after" };
+
 [Exposed=Window]
 interface AnimationEffect {
     EffectTiming         getTiming();