Disambiguate `this` and use the `@this` annotation in `media-editor.js`. This will become way more necessary in other places, but a few examples here show how confusing it can be. 

See #26870.


Built from https://develop.svn.wordpress.org/trunk@26989


git-svn-id: http://core.svn.wordpress.org/trunk@26866 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove debug cruft.

Props nacin.
See #26870.


Built from https://develop.svn.wordpress.org/trunk@26990


git-svn-id: http://core.svn.wordpress.org/trunk@26867 1a063a9b-81f0-0310-95a4-ce76da25c4cd