Document the Web Animation interfaces with spec links and TODOs

Source/core/animation/AnimationTimeline.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-animationtimeline-interface
+
+// TODO(dstockwell): This interface has several changes in the latest spec:
+// https://w3c.github.io/web-animations/#the-animationtimeline-interface
+
 [
     RuntimeEnabled=WebAnimationsAPI,
     WillBeGarbageCollected,
 ] interface AnimationTimeline {
     attribute double? currentTime;
     attribute double playbackRate;
     AnimationPlayer play(AnimationNode source);
     sequence<AnimationPlayer> getAnimationPlayers();
 };