Since `defaultPlaybackRate` is "used by the user agent when it exposes a user interface to the user", we should propagate this value to AVKit for fullscreen iOS video playback.
<rdar://problem/75012417>
Created attachment 422762 [details] Patch
Committed r274249: <https://commits.webkit.org/r274249> All reviewed patches have been landed. Closing bug and clearing flags on attachment 422762 [details].