Browser scrollbar is not hidden in Firefox when iframe is made visible after it's content has loaded

[suyash192]

Demo of issue:
https://codepen.io/anon/pen/ZyYXdZ

When the document is loaded inside an iframe and the iframe is made visible after it's content has been loaded then the browser scrollbar doesn't hide.

The getBrowserScrollSize function returns the scrollbar width and height as 0 in this case, and hence the scrollbar isn't hidden.

Firefox version: 53.0.3 (32-bit)