CSS: make the getStyles function more readable

(cherry-picked from 3a0d582cf63b6e8f77150d9c38d2bf34d0c7790e)

Closes gh-2393

src/css/curCSS.js

@@ -15,11 +15,13 @@ if ( window.getComputedStyle ) {
 		// Support: IE<=11+, Firefox<=30+ (#15098, #14150)
 		// IE throws on elements created in popups
 		// FF meanwhile throws on frame elements through "defaultView.getComputedStyle"
-		if ( elem.ownerDocument.defaultView.opener ) {
-			return elem.ownerDocument.defaultView.getComputedStyle( elem );
+		var view = elem.ownerDocument.defaultView;
+
+		if ( !view.opener ) {
+			view = window;
 		}
 
-		return window.getComputedStyle( elem );
+		return view.getComputedStyle( elem );
 	};
 
 	curCSS = function( elem, name, computed ) {