Fix dialog too small when iframe not initially visible

If the iframe is not initially visible, `boxSizingReliableVal` gets set to false, which triggers this code to set the wrong dialog width here:
https://github.com/jquery/jquery/blob/a7cc3a0f0418d168ff3818576161c5af24d0ca78/src/css.js#L418-L426

I think this should solve jquery/jquery-ui#2176.

src/css/support.js

@@ -17,6 +17,10 @@ define( [
 		if ( !div ) {
 			return;
 		}
+		// Don't run until window is visible (gh-2176)
+		if ( documentElement.offsetHeight === 0 ) {
+			return;
+		}
 
 		container.style.cssText = "position:absolute;left:-11111px;width:60px;" +
 			"margin-top:1px;padding:0;border:0";