Plupload wrapper: Call init callback on PostInit event.
Fixes the background image uploader in Customizer, which was broken after [27316].

fixes #27620.
Built from https://develop.svn.wordpress.org/trunk@27933


git-svn-id: http://core.svn.wordpress.org/trunk@27763 1a063a9b-81f0-0310-95a4-ce76da25c4cd