Introduce `maybe_regenerate_attachment_metadata( $attachment )`. On the Edit Media screen, call it for Audio and Video files.

The functions checks if the item is lacking metadata altogether. If a video or audio file was uploaded prior to 3.6, it does not have any metadata. This tries to fix it. Implements locking via a transient to protect against this running in parallel with another request.

This is the minimum viable product for #26825, but leaving the ticket open unless this function needs to be called in other places.

See #26825.



git-svn-id: https://develop.svn.wordpress.org/trunk@27127 602fd350-edb4-49c9-b593-d223f7449a82