[web-animations-1] Animations are current if non-idle and associated with non-increasing timelines. (w3c#8089)

If an animation is associated with a non-monotically increasing timeline it may become active in the future (if non-idle). As such it should still be considered current (the same as an animation which may play in the future). This fixes w3c#8053.

Author: flackr

web-animations-1/Overview.bs

@@ -2396,7 +2396,10 @@ of the following conditions are true:
     the [=animation effect=] is in the [=animation effect/before phase=], or
 *   the [=animation effect=] is [=associated with an animation=] with
     a [=playback rate=] < 0 and
-    the [=animation effect=] is in the [=animation effect/after phase=].
+    the [=animation effect=] is in the [=animation effect/after phase=], or
+*   the [=animation effect=] is [=associated with an animation=] not in the
+    [=play state/idle=] [=play state=] with a non-null associated
+    [=timeline=] that is not [=monotonically increasing=].
 
 An animation effect is <dfn>in effect</dfn> if its <a>active time</a>, as
 calculated according to the procedure in [[#calculating-the-active-time]],