Ensure Thickbox loading spinner is properly sized for HiDPI displays.

props paulwilde.
see #26952.

Built from https://develop.svn.wordpress.org/trunk@27678


git-svn-id: http://core.svn.wordpress.org/trunk@27521 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/common-rtl.css

@@ -2008,6 +2008,8 @@ body.index-php #TB_window {
 	body.plugin-install-php #TB_window,
 	body.index-php #TB_window {
 		background: #fcfcfc url( ../images/spinner-2x.gif ) no-repeat center;
+		-webkit-background-size: 20px 20px;
+		background-size: 20px 20px;
 	}
 }