fix(vttjs): wait till tech el in DOM before loading vttjs (#4177)

The issue is that in Flash, Flash.embed wraps the object element in a div but that object element is what is referenced by this.el(), so, parentNode isn't null but we aren't actually in the DOM. Instead, just check to see whether the tech element is in the DOM or not via node.contains().

src/js/tech/tech.js

@@ -515,7 +515,7 @@ class Tech extends Component {
     // Initially, Tech.el_ is a child of a dummy-div wait until the Component system
     // signals that the Tech is ready at which point Tech.el_ is part of the DOM
     // before inserting the WebVTT script
-    if (this.el().parentNode !== null && this.el().parentNode !== undefined) {
+    if (document.body.contains(this.el())) {
       const vtt = require('videojs-vtt.js');
 
       // load via require if available and vtt.js script location was not passed in