REST API: Restore access to audio/video metadata functions.

Ensures `wp_read_video_metadata()`/`wp_read_audio_metadata()` functions are available when uploading video and audio. Fixes error introduced in [43589].

Merges [43850] from the 5.0 branch to trunk.

Props ocean90.
See #43757.

Built from https://develop.svn.wordpress.org/trunk@44216


git-svn-id: https://core.svn.wordpress.org/trunk@44046 1a063a9b-81f0-0310-95a4-ce76da25c4cd