In `wp.Uploader`, when `uploader:ready` is triggered, set `ready` to `true` on the instance. This allows media workflows to check for an existing uploaded instance when opening.

Without this check, workflows might wait for `uploader:ready`, which could already have been fired. This would result in an unresponsive editor dropzone.

Fixes #29689.

Built from https://develop.svn.wordpress.org/trunk@29917


git-svn-id: http://core.svn.wordpress.org/trunk@29669 1a063a9b-81f0-0310-95a4-ce76da25c4cd