WP_Debug_Data: Extract wp-media data into separate method.

This is the sixth part in a larger modularization of the data in `WP_Debug_Data`. Previously this was a single massive method drawing in debug data from various groups of related data, where the groups were independent from each other.

This patch separates the sixth of twelve groups, the `wp-media` info, into a separate method focused on that data.

This work precedes changes to make the `WP_Debug_Data` class more extensible for better use by plugin and theme code.

Developed in WordPress/wordpress-develop#7356
Discussed in https://core.trac.wordpress.org/ticket/61648

Props apermo, dmsnell.
See #61648.

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git-svn-id: https://core.svn.wordpress.org/trunk@58456 1a063a9b-81f0-0310-95a4-ce76da25c4cd