[web-animations-1] Maintain start time when changing playback rate on non-monotonic timeline (w3c#8090)

Author: flackr

web-animations-1/Overview.bs

@@ -1846,7 +1846,8 @@ is as follows:
 
 1.  Set the [=playback rate=] to <var>new playback rate</var>.
 
-1.  If <var>previous time</var> is <a lt="unresolved">resolved</a>,
+1.  If the <var>animation</var> is associated with a [=monotonically increasing=]
+    [=timeline=] and <var>previous time</var> is <a lt="unresolved">resolved</a>,
     <a>set the current time</a> of <var>animation</var> to
     <var>previous time</var>