Editor: Fix determining TinyMCE editor width when in inline mode instead of iframe mode.

See #40854.
Amends [41985].
Props aduth.
Fixes #42416 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42390


git-svn-id: http://core.svn.wordpress.org/trunk@42219 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/mce-view.js

@@ -863,7 +863,7 @@
 
 			// Obtain the target width for the embed.
 			if ( self.editor ) {
-				maxwidth = self.editor.iframeElement.clientWidth - 20; // Minus the sum of horizontal margins and borders.
+				maxwidth = self.editor.getBody().clientWidth;
 			}
 
 			wp.ajax.post( this.action, {

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-42389';
+$wp_version = '5.0-alpha-42390';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.