KeyframeTrack: Fix toJSON static override check

  Subclasses of KeyframeTrack classes will never have undefined static toJSON

src/animation/KeyframeTrack.js

@@ -34,7 +34,7 @@ class KeyframeTrack {
 		let json;
 
 		// derived classes can define a static toJSON method
-		if ( trackType.toJSON !== undefined ) {
+		if ( trackType.toJSON !== this.toJSON ) {
 
 			json = trackType.toJSON( track );