Use IDL union for Timing.duration

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 1bb3198c61034c415503320bccd5377b8c66de40..c1e8ee7fa809bd058131a47b78eee5b65924eca8 100644
--- a/Source/core/animation/Timing.idl
+++ b/Source/core/animation/Timing.idl
@@ -12,17 +12,7 @@
     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;