swift修改webview上下滚动出边界的的背景黑色

 webView.backgroundColor = UIColor.clearColor()
        for subView in webView.subviews {
            if subView.isKindOfClass(UIScrollView) {
                for shadow in subView.subviews {
                    if shadow.isKindOfClass(UIImageView){
                        shadow.hidden = true
                    }
                }
            }

        }

必须要设置backgroundColor = UIColor.clearColor() 不然也是没效果的