Document the Web Animation interfaces with spec links and TODOs

Source/core/animation/ElementAnimation.idl

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
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  * modification, are permitted provided that the following conditions are
  * met:
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  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 10 matching lines @@
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  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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-// http://www.w3.org/TR/web-animations/#idl-def-Element
-// FIXME: move to Animatable
-// http://dev.w3.org/fxtf/web-animations/#idl-def-Animatable
+// http://www.w3.org/TR/web-animations/#the-animatable-interface
+// http://www.w3.org/TR/web-animations/#extensions-to-the-element-interface
+
+// TODO(dstockwell): This should be an Animatable interface, where Element
+// implements Animatable. The interface also has several changes:
+// https://w3c.github.io/web-animations/#the-animatable-interface
 
 partial interface Element {
     // FIXME: Union types with dictionary type members doesn't work (yet).
     // AnimationPlayer animate((AnimationEffect or sequence<Dictionary>)? effect, optional (double or AnimationTimingProperties) timing);
     [RaisesException] AnimationPlayer animate((AnimationEffect or sequence<Dictionary>)? effect, optional double timing);
     [RaisesException] AnimationPlayer animate((AnimationEffect or sequence<Dictionary>)? effect, AnimationTimingProperties timing);
     [RuntimeEnabled=WebAnimationsAPI] sequence<AnimationPlayer> getAnimationPlayers();
 };