Add TODOs to remove AnimationPlayerEvent and AnimationPlayerEventInit

Source/core/events/AnimationPlayerEventInit.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // http://www.w3.org/TR/web-animations/#the-animationplayerevent-interface
 
+// TODO(dstockwell): This dictionary has been removed in the latest spec:
+// https://w3c.github.io/web-animations/
+
 dictionary AnimationPlayerEventInit : EventInit {
     double? currentTime = null;
     double? timelineTime = null;
 };