Make AnimationPlayer.startTime nullable

Author: birtles

web-animations/index.html

@@ -5658,7 +5658,7 @@ <h3>The <code>AnimationPlayer</code> interface</h3>
           <dd>
             The <a>timeline</a> associated with this player.
           </dd>
-          <dt>attribute double startTime</dt>
+          <dt>attribute double? startTime</dt>
           <dd>
             Returns the <a title="player start time">start time</a> of this
             player.