Upgrader: Use always `window.postMessage` for decrement of update count bubbles when `IFRAME_REQUEST` is defined.
Fixes decrement of translation update count bubbles when updated through the bulk upgrader.

fixes #29054.
Built from https://develop.svn.wordpress.org/trunk@29357


git-svn-id: http://core.svn.wordpress.org/trunk@29133 1a063a9b-81f0-0310-95a4-ce76da25c4cd