Document the Web Animation interfaces with spec links and TODOs

Source/core/animation/Timing.idl

Index: Source/core/animation/Timing.idl
diff --git a/Source/core/animation/Timing.idl b/Source/core/animation/Timing.idl
index a762527ff50ad7c825b6357b414bcef10e4bdc1c..6cd5e975c826331a9748d1ff9ad2b615497042b5 100644
--- a/Source/core/animation/Timing.idl
+++ b/Source/core/animation/Timing.idl
@@ -2,6 +2,12 @@
 // 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-animationtiming-interface
+
+// TODO(dstockwell): This interface does not exist in the latest spec, which
+// instead has a AnimationEffectTimingReadonly interface:
+// https://w3c.github.io/web-animations/#the-animationeffecttimingreadonly-interface
+
 [
     ImplementedAs=AnimationNodeTiming,
     RuntimeEnabled=WebAnimationsAPI,