Use IDL union for Timing.duration

Source/core/animation/Timing.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.
 
 [
     RuntimeEnabled=WebAnimationsAPI,
     ImplementedAs=AnimationNodeTiming,
     WillBeGarbageCollected,
 ] interface Timing {
     attribute double delay;
     attribute double endDelay;
     attribute DOMString fill;
     attribute double iterationStart;
     attribute double iterations;
-
-    // FIXME: This uses a NamedPropertyGetter to implement the 'duration' attribute
-    // because duration has a union type (which is tricky to do with an attribute).
-    // Fix will be in a follow-up patch if there is a better solution.
-    [NotEnumerable, ImplementedAs=getDuration] getter (double or DOMString) (DOMString name);
-
-    // FIXME: If the user calls animation.specified.duration = "" (empty string) then duration
-    // gets set to 0 (This is correct behavior for IDL). Correct result is for duration to
-    // be set to 'auto'.
-    [TypeChecking=Interface, ImplementedAs=setDuration] setter double (DOMString name, double duration);
-
+    attribute (double or DOMString) duration;
     attribute double playbackRate;
     attribute DOMString direction;
     attribute DOMString easing;
 };