Fix MP4 text parser integration

* removes an assertion which is no longer valid
 * corrects the registration of the VTT in MP4 parser
 * fixes some type issues in TextEngine

Change-Id: I0b7f41f01e5d7be2932c86a56f3cdfd6a53d04cb

lib/media/media_source_engine.js

@@ -352,8 +352,6 @@ shaka.media.MediaSourceEngine.prototype.getBuffered_ = function(contentType) {
 shaka.media.MediaSourceEngine.prototype.appendBuffer =
     function(contentType, data, startTime, endTime) {
   if (contentType == 'text') {
-    goog.asserts.assert(startTime != null && endTime != null,
-                        'text streams do not have init segments!');
     return this.textEngine_.appendBuffer(data, startTime, endTime);
   }
   return this.enqueueOperation_(

lib/media/mp4_vtt_parser.js

@@ -193,4 +193,4 @@ shaka.media.Mp4VttParser.BOX_TYPE_STTG = 0x73747467;
 
 
 shaka.media.TextEngine.registerParser(
-    'application/mp4; codecs="wvtt"', shaka.media.Mp4TtmlParser);
+    'application/mp4; codecs="wvtt"', shaka.media.Mp4VttParser);

lib/media/text_engine.js

@@ -59,7 +59,7 @@ shaka.media.TextEngine = function(track, mimeType) {
 /**
  * Parses a text buffer into an array of cues.
  *
- * @typedef {function(ArrayBuffer, number, number):!Array.<!TextTrackCue>}
+ * @typedef {function(ArrayBuffer, ?number, ?number):!Array.<!TextTrackCue>}
  * @exportDoc
  */
 shaka.media.TextEngine.TextParser;
@@ -112,8 +112,8 @@ shaka.media.TextEngine.prototype.destroy = function() {
 
 /**
  * @param {ArrayBuffer} buffer
- * @param {number} startTime
- * @param {number} endTime
+ * @param {?number} startTime
+ * @param {?number} endTime
  * @return {!Promise}
  */
 shaka.media.TextEngine.prototype.appendBuffer =
@@ -125,6 +125,14 @@ shaka.media.TextEngine.prototype.appendBuffer =
     // Parse the buffer and add the new cues.
     var cues = this.parser_(buffer, startTime, endTime);
 
+    if (!cues.length) {
+      // Init segments have no cues and no times.
+      return;
+    }
+    // If cues were parsed, startTime and endTime should be non-null.
+    goog.asserts.assert(startTime != null && endTime != null,
+                        'start and end times should be non-null!');
+
     for (var i = 0; i < cues.length; ++i) {
       cues[i].startTime += offset;
       cues[i].endTime += offset;