fix: Fix exception enabling captions on HLS (shaka-project#4894)

Broken by PR shaka-project#4824

Fixes shaka-project#4889

lib/media/media_source_engine.js

@@ -920,8 +920,16 @@ shaka.media.MediaSourceEngine = class {
    * @return {!Promise}
    */
   async resync(contentType, timestampOffset) {
+    const ContentType = shaka.util.ManifestParserUtils.ContentType;
+
     goog.asserts.assert(this.sequenceMode_,
         'resync only used with sequence mode!');
+
+    if (contentType == ContentType.TEXT) {
+      // This operation is for audio and video only.
+      return;
+    }
+
     // Queue an abort() to help MSE splice together overlapping segments.
     // We set appendWindowEnd when we change periods in DASH content, and the
     // period transition may result in overlap.