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.


git-svn-id: https://develop.svn.wordpress.org/trunk@29917 602fd350-edb4-49c9-b593-d223f7449a82