Web Animations: Enable additive animations in experimental

Source/core/animation/ElementAnimation.idl

Index: Source/core/animation/ElementAnimation.idl
diff --git a/Source/core/animation/ElementAnimation.idl b/Source/core/animation/ElementAnimation.idl
index 7129e4df550d52514719c6c29198dbf00fa6a0a1..a6ac513a7e9f29242cc774f1c36fbd11db1cc217 100644
--- a/Source/core/animation/ElementAnimation.idl
+++ b/Source/core/animation/ElementAnimation.idl
@@ -37,8 +37,8 @@
 
 partial interface Element {
     // FIXME: Union types with dictionary type members doesn't work (yet).
-    // Animation animate((EffectModel or sequence<Dictionary>)? effect, optional (double or KeyframeEffectOptions) timing);
-    [Measure, RaisesException] Animation animate((EffectModel or sequence<Dictionary>)? effect, optional double timing);
-    [Measure, RaisesException] Animation animate((EffectModel or sequence<Dictionary>)? effect, KeyframeEffectOptions timing);
+    // Animation animate((EffectModel or sequence<Dictionary> or Dictionary)? effect, optional (double or KeyframeEffectOptions) timing);
+    [Measure, RaisesException] Animation animate((EffectModel or sequence<Dictionary> or Dictionary)? effect, optional double timing);
+    [Measure, RaisesException] Animation animate((EffectModel or sequence<Dictionary> or Dictionary)? effect, KeyframeEffectOptions timing);
     [RuntimeEnabled=WebAnimationsAPI] sequence<Animation> getAnimations();
 };