Decrement of translation update count bubbles doesn't work, when updated through iframe

[ocean90]

Source snippet from the iframe:

// ...
<script type="text/javascript">
	if ( window.postMessage && JSON ) {
		window.parent.postMessage( JSON.stringify( { action: "decrementUpdateCount", upgradeType: "plugin" } ), window.location.protocol + "//" + window.location.hostname );
	}
</script>
// ...
<div class="update-messages lp-show-latest">
	<h4>Updating translations for WordPress (ar)&#8230;</h4>
	// ...
</div>
<script type="text/javascript">
	(function( wp ) {
		if ( wp && wp.updates.decrementCount ) {
			wp.updates.decrementCount( "translation" );
		}
	})( window.wp );
</script>

The second one should use postMessage too.

Related: #17703

[ocean90]

In 29357:

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.