Move upgrader_process_complete for core to its proper place in Core_Upgrader.

This means it will be firing as a JS redirect is taking place if the update is from pre-3.4. Acceptable.

props dd32.
fixes #25659.


git-svn-id: https://develop.svn.wordpress.org/trunk@25861 602fd350-edb4-49c9-b593-d223f7449a82