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.

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git-svn-id: http://core.svn.wordpress.org/trunk@25773 1a063a9b-81f0-0310-95a4-ce76da25c4cd