The ResizeManager's iframe element is able to be focused via tabbing, which results in a bad user experience for users that rely on a screen reader to navigate the video and its sibling elements. The fix is to set the tabIndex to "-1", and the aria-hidden property to true for good measure.
Use ResizeObserver when available for better and more performant resizing information, otherwise, fall back to a throttled resize event on an iframe that's the size of the player.
Allows a video.js user to disable this by setting resizeManager: false as an option since the component will not be initialized.
Add a debounce util.
This reverts #4800 (e0ed0b5) because we end up getting two playerresize events with the dimension methods now.
