[FIX] mass_mailing: update iframe size on pick template

When picking a mailing template, the size of the contents of the iframe
changes but we failed to signal it so the iframe could resize as well.

closes odoo/odoo#125740

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/static/src/js/mass_mailing_html_field.js

@@ -390,6 +390,7 @@ export class MassMailingHtmlField extends HtmlField {
                 }
                 // mark selection done for tour testing
                 $editable.addClass('theme_selection_done');
+                this.onIframeUpdated();
             }, 0);
         });