Document the Web Animation interfaces with spec links and TODOs

Source/core/animation/AnimationEffect.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-animationeffect-interface
+
+// TODO(dstockwell): This interface has been removed in the latest spec:
+// https://w3c.github.io/web-animations/
+
 [
     RuntimeEnabled=WebAnimationsAPI,
     NoInterfaceObject,
     WillBeGarbageCollected
 ] interface AnimationEffect {
 };