[web-animations-2] Update timeline currentTime type to allow CSSNumericValue timelines (#8371)

Author: kevers-google

scroll-animations-1/Overview.bs

@@ -270,7 +270,7 @@ spec: cssom-view-1; type: dfn;
 	or if there is no [=scrollable overflow=],
 	then the {{ScrollTimeline}} is [=inactive timeline|inactive=].
 
-	A {{ScrollTimeline}}’s {{EffectTiming/duration}} is 100%.
+	A {{ScrollTimeline}}’s {{AnimationTimeline/duration}} is 100%.
 
 ## Named Scroll Progress Timelines ## {#scroll-timelines-named}
 

web-animations-2/Overview.bs

@@ -2178,13 +2178,23 @@ Items sorted earlier are executed before those sorted later.
 <pre class="idl">
 [Exposed=Window]
 partial interface AnimationTimeline {
+    readonly attribute CSSNumberish? currentTime;
     readonly attribute CSSNumberish? duration;
     Animation play (optional AnimationEffect? effect = null);
 };
 </pre>
 
 <div class='attributes'>
 
+Update the attribute type for currentTime.
+
+:   <dfn attribute for=AnimationTimeline>currentTime</dfn>
+::  Returns the <a lt="timeline current time">current time</a> for this
+    timeline or <code>null</code> if this timeline is
+    <a lt="inactive timeline">inactive</a>.  The value is expressed as a
+    percentage for a [=progress-based timeline=], or as a double in
+    milliseconds otherwise.
+
 :   <dfn attribute for=AnimationTimeline>duration</dfn>
 ::  Returns the <a lt="timeline duration">duration</a> for this timeline.