Media Grid: expose the `$.Deffered()` object in `media.view.AttachmentsBrowser.updateContent()` so the grid can call `Backbone.history.start()` when models actually exist. 

This is a lot better then `_.delay( this.pray, 1000 )`, but needs continued study.

See #29052.

Built from https://develop.svn.wordpress.org/trunk@29420


git-svn-id: http://core.svn.wordpress.org/trunk@29198 1a063a9b-81f0-0310-95a4-ce76da25c4cd