[ONEM-26034] Use |innerText| for accessing dash.js subs content

HTML node property |textContent| returns text without any whitespaces.
It leads to |DASH-FMP4-AVC1-AAC-TTML Subtitles| failures on dash.js,
because the returned value is verified against expected text which
inluced '\n' character.

Usage of |innerText| property solves the above issue.

src/engines.js

@@ -34,7 +34,7 @@ function extractSubtitleCue(content, callback) {
       var subtitlesDiv = document.getElementById("subtitles");
       var elements = subtitlesDiv.getElementsByTagName("div");
       for (var i = 0; i < elements.length; i++) {
-        callback(elements[i].textContent);
+        callback(elements[i].innerText);
       }
     } else {
       for (var i = 0; i < cues.length; i++) {