Fix TTML positioning and cue externs (#594)

A number of TextTrackCue properties were not included in the externs and caused VTT default values to be used instead.  When converting TTML tts:origin values, we need to set VTTCue.snapToLines to false to indicate that cue.line is a percentage value.

externs/texttrack.js

@@ -33,3 +33,15 @@ TextTrackCue.prototype.positionAlign;
 
 /** @type {string} */
 TextTrackCue.prototype.lineAlign;
+
+
+/** @type {number|null|string} */
+TextTrackCue.prototype.line;
+
+
+/** @type {string} */
+TextTrackCue.prototype.vertical;
+
+
+/** @type {boolean} */
+TextTrackCue.prototype.snapToLines;

lib/media/ttml_text_parser.js

@@ -336,6 +336,12 @@ shaka.media.TtmlTextParser.addStyle_ = function(
         cue.position = Number(results[1]);
         cue.line = Number(results[2]);
       }
+      // A boolean indicating whether the line is an integer
+      // number of lines (using the line dimensions of the first
+      // line of the cue), or whether it is a percentage of the
+      // dimension of the video. The flag is set to true when lines
+      // are counted, and false otherwise.
+      cue.snapToLines = false;
     }
   }