Media: Add object-fit to the allowed list of CSS properties.

This resolves a bug in Featured Image blocks where `object-fit` was being removed during the `render_callback`.

Props raduiason, pbiron, kebbet, SergeyBiryukov, bernhard-reiter, ironprogrammer, xknown, audrasjb, ckanderson22, ivanjeronimo, seriouslysenpai, davidbaumwald.
Merges [54675] to the 5.9 branch.
Fixes #56855.



git-svn-id: https://develop.svn.wordpress.org/branches/5.9@54764 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/kses.php

@@ -2415,6 +2415,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'direction',
 			'float',
 			'list-style-type',
+			'object-fit',
 			'object-position',
 			'overflow',
 			'vertical-align',

tests/phpunit/tests/kses.php

@@ -1093,6 +1093,11 @@ EOF;
 				'css'      => 'background: conic-gradient(at 0% 30%, red 10%, yellow 30%, #1e90ff 50%)',
 				'expected' => 'background: conic-gradient(at 0% 30%, red 10%, yellow 30%, #1e90ff 50%)',
 			),
+			// `object-fit` introduced in 6.1.
+			array(
+				'css'      => 'object-fit: cover',
+				'expected' => 'object-fit: cover',
+			),
 			// Expressions are not allowed.
 			array(
 				'css'      => 'height: expression( body.scrollTop + 50 + "px" )',