Add TODOs to remove AnimationPlayerEvent and AnimationPlayerEventInit

Source/core/events/AnimationPlayerEventInit.idl

Index: Source/core/events/AnimationPlayerEventInit.idl
diff --git a/Source/core/events/AnimationPlayerEventInit.idl b/Source/core/events/AnimationPlayerEventInit.idl
index b1191c51c1a123cc2c0bb597b8e9d3d93aed1228..f77620da65a2b9493cefd51b4e95da331ffa24db 100644
--- a/Source/core/events/AnimationPlayerEventInit.idl
+++ b/Source/core/events/AnimationPlayerEventInit.idl
@@ -4,6 +4,9 @@
 
 // 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;