Document the Web Animation interfaces with spec links and TODOs

Source/core/animation/AnimationTimingProperties.idl

 // Copyright 2015 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.
 
+// https://w3c.github.io/web-animations/#the-animationtiminginput-dictionary
+
+// TODO(dstockwell): Rename this to AnimationEffectTimingProperties.
 dictionary AnimationTimingProperties {
     double                             delay = 0;
     double                             endDelay = 0;
+    // TODO(dstockwell): The fill type should be FillMode.
     DOMString                          fill = "auto";
     double                             iterationStart = 0.0;
     unrestricted double                iterations = 1.0;
     (unrestricted double or DOMString) duration = "auto";
+    // TODO(dstockwell): There is no playbackRate member in the spec.
     double                             playbackRate = 1.0;
+    // TODO(dstockwell): The direction type should be PlaybackDirection.
     DOMString                          direction = "normal";
     DOMString                          easing = "linear";
 };