WebBrowser iframe not displays when using HttpWebRequest

I use Winform WebBrowser to load an website. The website is contains an iframe. The URL here: http://ift.tt/1v05tFk

When i use Navigate() method to load that URL. The WebBrowser display an texteditor and an iframe.

When i try to use HttpWebRequest to load the website, like this:

    HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create("http://ift.tt/1v05tFk");

    HttpWebResponse myResponse = (HttpWebResponse)myRequest.GetResponse();

    webBrowser1.DocumentStream = myResponse.GetResponseStream();

The WebBrowser is also display an html's texteditor, but the iframe is not displayed. It just show me the iframe's URL. Why the WebBrowser not display the iframe contents ? While the HTML,CSS,JS still work, but the Iframe not ?