StringKeyframeTrack: Convert to es6 class

src/animation/tracks/StringKeyframeTrack.js

@@ -4,16 +4,9 @@ import { KeyframeTrack } from '../KeyframeTrack.js';
 /**
  * A Track that interpolates Strings
  */
+class StringKeyframeTrack extends KeyframeTrack {}
 
-function StringKeyframeTrack( name, times, values, interpolation ) {
-
-	KeyframeTrack.call( this, name, times, values, interpolation );
-
-}
-
-StringKeyframeTrack.prototype = Object.assign( Object.create( KeyframeTrack.prototype ), {
-
-	constructor: StringKeyframeTrack,
+Object.assign( StringKeyframeTrack.prototype, {
 
 	ValueTypeName: 'string',
 	ValueBufferType: Array,