Remove some obsolete entries in Web Animations IDL

third_party/WebKit/Source/core/animation/AnimationEffectTiming.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-animationeffecttiming-interface
 // TODO(dstockwell): Should extend AnimationEffectTimingReadOnly
 
 [
     RuntimeEnabled=WebAnimationsAPI,
 ] interface AnimationEffectTiming {
     attribute double delay;
     attribute double endDelay;
     attribute DOMString fill;
     [RaisesException=Setter] attribute double iterationStart;
     [RaisesException=Setter] attribute unrestricted double iterations;
     [RaisesException=Setter] attribute (unrestricted double or DOMString) duration;
-    // TODO(dstockwell): playbackRate has been removed from level 1.
-    attribute double playbackRate;
     attribute DOMString direction;
     [RaisesException=Setter] attribute DOMString easing;
 };