fix: Check for VTTCue (#8370)

package.json

@@ -98,7 +98,7 @@
     "safe-json-parse": "4.0.0",
     "videojs-contrib-quality-levels": "4.0.0",
     "videojs-font": "4.1.0",
-    "videojs-vtt.js": "0.15.4"
+    "videojs-vtt.js": "0.15.5"
   },
   "devDependencies": {
     "@babel/core": "^7.9.0",

src/js/tracks/text-track.js

@@ -391,7 +391,7 @@ class TextTrack extends Track {
   addCue(originalCue) {
     let cue = originalCue;
 
-    if (window.vttjs && !(originalCue instanceof window.vttjs.VTTCue)) {
+    if (cue.constructor && cue.constructor.name !== 'VTTCue') {
       cue = new window.vttjs.VTTCue(originalCue.startTime, originalCue.endTime, originalCue.text);
 
       for (const prop in originalCue) {

test/unit/tracks/text-track.test.js

@@ -231,6 +231,21 @@ QUnit.test('original cue can be used to remove cue from cues list', function(ass
   assert.equal(tt.cues.length, 0, 'we have removed cue1');
 });
 
+QUnit.test('non-VTTCue can be used to remove cue from cues list', function(assert) {
+  const tt = new TextTrack({
+    tech: this.tech
+  });
+
+  const cue1 = { id: 1, text: 'test' };
+
+  assert.equal(tt.cues.length, 0, 'start with zero cues');
+  tt.addCue(cue1);
+  assert.equal(tt.cues.length, 1, 'we have one cue');
+
+  tt.removeCue(cue1);
+  assert.equal(tt.cues.length, 0, 'we have removed cue1');
+});
+
 QUnit.test('can only remove one cue at a time', function(assert) {
   const tt = new TextTrack({
     tech: this.tech