fix(HLS): Fix subtitle timing (#7625)

Simplifies subtitle management for sequence mode and segments mode 
Runs subtitle tests in segments mode as well (Tizen 3.0)

Fixes: https://github.com/shaka-project/shaka-player/issues/7447

lib/text/vtt_text_parser.js

@@ -86,13 +86,7 @@ shaka.text.VttTextParser = class {
     // Only use 'X-TIMESTAMP-MAP' with HLS.  This overrides offset above.
     if (blocks[0].includes('X-TIMESTAMP-MAP') &&
         this.manifestType_ == shaka.media.ManifestParser.HLS) {
-      if (this.sequenceMode_) {
-        // Compute a different, rollover-based offset for sequence mode.
-        offset = this.computeHlsSequenceModeOffset_(blocks[0], time);
-      } else {
-        // Calculate the offset from the segment startTime.
-        offset = time.segmentStart;
-      }
+      offset = this.computeHlsOffset_(blocks[0], time);
     }
 
     // Parse VTT regions.
@@ -129,7 +123,7 @@ shaka.text.VttTextParser = class {
    * @return {number}
    * @private
    */
-  computeHlsSequenceModeOffset_(headerBlock, time) {
+  computeHlsOffset_(headerBlock, time) {
     // https://bit.ly/2K92l7y
     // The 'X-TIMESTAMP-MAP' header is used in HLS to align text with
     // the rest of the media.