Menus: Account for browsers that set a default margin for buttons.

Fixes #32733.


Built from https://develop.svn.wordpress.org/trunk@33410


git-svn-id: http://core.svn.wordpress.org/trunk@33378 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-nav-menus-rtl.css

@@ -304,6 +304,7 @@
 	border: none;
 	color: #555;
 	cursor: pointer;
+	margin: 0;
 	padding: 20px;
 	position: absolute;
 	left: 31px;

wp-admin/css/customize-nav-menus.css

@@ -304,6 +304,7 @@
 	border: none;
 	color: #555;
 	cursor: pointer;
+	margin: 0;
 	padding: 20px;
 	position: absolute;
 	right: 31px;