Ignore file extension for HLS Subtitle Renditions

According to the spec, subtitle renditions must be Webvtt media
segments.

Issue:#2025
Issue:#2355

-------------
Created by MOE: https:/google/moe
MOE_MIGRATED_REVID=145289266

library/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaChunk.java

@@ -187,7 +187,7 @@ public boolean isLoadCanceled() {
  public void load() throws IOException, InterruptedException {
  if (extractor == null && !isPackedAudio) {
  // See HLS spec, version 20, Section 3.4 for more information on packed audio extraction.
- extractor = buildExtractorByExtension();
+ extractor = createExtractor();
  }
  maybeLoadInitData();
  if (!loadCanceled) {
@@ -329,11 +329,12 @@ private static DataSource buildDataSource(DataSource dataSource, byte[] encrypti
  return new Aes128DataSource(dataSource, encryptionKey, encryptionIv);
  }
 
- private Extractor buildExtractorByExtension() {
- // Set the extractor that will read the chunk.
+ private Extractor createExtractor() {
+ // Select the extractor that will read the chunk.
  Extractor extractor;
  boolean usingNewExtractor = true;
- if (lastPathSegment.endsWith(WEBVTT_FILE_EXTENSION)
+ if (MimeTypes.TEXT_VTT.equals(hlsUrl.format.sampleMimeType)
+ || lastPathSegment.endsWith(WEBVTT_FILE_EXTENSION)
  || lastPathSegment.endsWith(VTT_FILE_EXTENSION)) {
  extractor = new WebvttExtractor(trackFormat.language, timestampAdjuster);
  } else if (!needNewExtractor) {