fix(accessibility): frame must have a title attribute (#7754)

Author: gjanblaszczyk

Diff for: lang/en.json

@@ -85,5 +85,6 @@
   "End of dialog window.": "End of dialog window.",
   "{1} is loading.": "{1} is loading.",
   "Exit Picture-in-Picture": "Exit Picture-in-Picture",
-  "Picture-in-Picture": "Picture-in-Picture"
+  "Picture-in-Picture": "Picture-in-Picture",
+  "No content": "No content"
 }

Diff for: src/js/resize-manager.js

@@ -92,7 +92,8 @@ class ResizeManager extends Component {
   createEl() {
     return super.createEl('iframe', {
       className: 'vjs-resize-manager',
-      tabIndex: -1
+      tabIndex: -1,
+      title: this.localize('No content')
     }, {
       'aria-hidden': 'true'
     });