How to close webview.stage

Im using this code its working but im not able to close the webview stage. How can I close the webview stage, using on button click?

package  {

    import flash.display.MovieClip;

    import flash.media.StageWebView;

    import flash.geom.Rectangle;

    import flash.events.KeyboardEvent;

    import flash.ui.Keyboard;

    import flash.desktop.NativeApplication;

    public class mhtml extends MovieClip{

        private var webView:StageWebView = new StageWebView();

        public function mhtml()

        {

            webView.stage = this.stage;

            webView.viewPort = new Rectangle( 50, 50, 700, 400 );

            webView.loadURL( "http://delta-adv.com/roche/form.php" );

            stage.addEventListener( KeyboardEvent.KEY_DOWN, onKey );

        }

        private function onKey( event:KeyboardEvent ):void

        {

            if( event.keyCode == Keyboard.BACK && webView.isHistoryBackEnabled )

            {

                trace("Back.");

                webView.historyBack();

                event.preventDefault();

            }

            if( event.keyCode == Keyboard.SEARCH && webView.isHistoryForwardEnabled )

            {

                trace("Forward.");

                webView.historyForward();

            }

        }

    }

}