Document the Web Animation interfaces with spec links and TODOs

Source/core/animation/ComputedTimingProperties.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.
 
+// https://w3c.github.io/web-animations/#the-computedtimingproperties-dictionary
+
 dictionary ComputedTimingProperties : AnimationTimingProperties {
+    // TODO(dstockwell): There is no startTime member in the spec.
     double              startTime;
     unrestricted double endTime;
     unrestricted double activeDuration;
     double?             localTime;
+    // TODO(dstockwell): The timeFraction and currentIteration members should
+    // both be of type unrestricted double? (nullable).
     unrestricted double timeFraction;
     unsigned long?      currentIteration;
 };