Use window.getComputedStyle to prevent breaking jsdom/node.js . Fixes…

… #12235

src/css.js

@@ -279,11 +279,11 @@ jQuery.extend({
 });
 
 // NOTE: To any future maintainer, we've used both window.getComputedStyle
-// and getComputedStyle here to produce a better gzip size
+// because jsdom on node.js will break without it.
 if ( window.getComputedStyle ) {
 	curCSS = function( elem, name ) {
 		var ret, width, minWidth, maxWidth,
-			computed = getComputedStyle( elem, null ),
+			computed = window.getComputedStyle( elem, null ),
 			style = elem.style;
 
 		if ( computed ) {