CSS: simplify "defaultDisplay" module

Closes gh-1962

Author: markelog

src/css/defaultDisplay.js

@@ -49,7 +49,7 @@ function defaultDisplay( nodeName ) {
 				.appendTo( doc.documentElement );
 
 			// Always write a new HTML skeleton so Webkit and Firefox don't choke on reuse
-			doc = ( iframe[ 0 ].contentWindow || iframe[ 0 ].contentDocument ).document;
+			doc = iframe[ 0 ].contentDocument;
 
 			// Support: IE
 			doc.write();