feat: update videojs-vtt.js and wrap native cues in TextTrack (#4115)

Update videojs-vtt.js and don't auto-export its versions of VTTCue globally.
When our TextTrack object is given a cue, if it's a native cue, wrap it in our emulated vttjs.VTTCue.

Fixes #4093.

package.json

@@ -44,7 +44,7 @@
     "tsml": "1.0.1",
     "videojs-font": "2.0.0",
     "videojs-ie8": "1.1.2",
-    "videojs-vtt.js": "0.12.1",
+    "videojs-vtt.js": "0.12.2",
     "xhr": "2.3.3"
   },
   "devDependencies": {

src/js/tech/tech.js

@@ -520,9 +520,6 @@ class Tech extends Component {
       // as an option. novtt builds will turn the above require call into an empty object
       // which will cause this if check to always fail.
       if (!this.options_['vtt.js'] && isPlain(vtt) && Object.keys(vtt).length > 0) {
-        Object.keys(vtt).forEach(function(k) {
-          window[k] = vtt[k];
-        });
         this.trigger('vttjsloaded');
         return;
       }

src/js/tracks/text-track.js

@@ -336,7 +336,17 @@ class TextTrack extends Track {
    * @param {TextTrack~Cue} cue
    *        The cue to add to our internal list
    */
-  addCue(cue) {
+  addCue(originalCue) {
+    let cue = originalCue;
+
+    if (!(originalCue instanceof window.vttjs.VTTCue)) {
+      cue = new window.vttjs.VTTCue(originalCue.startTime, originalCue.endTime, originalCue.text);
+
+      for (const prop in originalCue) {
+        cue[prop] = originalCue[prop];
+      }
+    }
+
     const tracks = this.tech_.textTracks();
 
     for (let i = 0; i < tracks.length; i++) {