Bug 1483404 - Use "auto" values for unspecified keyframe-specific composite operations; r=smaug

Summary:
This brings our implementation into line with the following spec change:

  ced6b5aac0

See https://github.com/w3c/csswg-drafts/issues/3000

Reviewers: smaug

Tags: #secure-revision

Bug #: 1483404

Differential Revision: https://phabricator.services.mozilla.com/D3384

dom/animation/KeyframeEffect.cpp

@@ -1099,9 +1099,8 @@ KeyframeEffect::GetKeyframes(JSContext*& aCx,
       keyframe.mTimingFunction.ref().AppendToString(keyframeDict.mEasing);
     } // else if null, leave easing as its default "linear".
 
-    if (keyframe.mComposite) {
-      keyframeDict.mComposite.SetValue(keyframe.mComposite.value());
-    }
+    keyframeDict.mComposite =
+      KeyframeUtils::ToCompositeOperationOrAuto(keyframe.mComposite);
 
     JS::Rooted<JS::Value> keyframeJSValue(aCx);
     if (!ToJSValue(aCx, keyframeDict, &keyframeJSValue)) {