get mpeg4 url from youtube if UA = iPad

var xhr = new XMLHttpRequest();
xhr.open("get", "http://m.youtube.com/watch?ajax=1&layout=mobile&tsp=1&utcoffset=540&v=Dbydyibcfi8&preq=");
// xhr.setRequestHeader("User-Agent", "Mozilla/5.0(iPad; U; CPU OS 4_3 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8F191 Safari/6533.18.5")
xhr.onload = function(e) {
  console.log(JSON.parse(e.target.responseText.slice(4)).content.video.fmt_stream_map[1].url);
//  window.open(JSON.parse(e.target.responseText.slice(4)).content.video.fmt_stream_map[1].url);
}
xhr.send();


/*
// http://developer.chrome.com/extensions/webRequest.html


var UA = null;
var description = null;

var requestFilter = {
    urls: ["<all_urls>"]
},
extraInfoSpec = ['requestHeaders', 'blocking'],

handler = function (details) {

    if (UA == null) {
        return;
    }
    var headers = details.requestHeaders,
    blockingResponse = {};

    for (var i = 0, l = headers.length; i < l; ++i) {
        if (headers[i].name == 'User-Agent') {
            headers[i].value = UA;
            break;
        }
    }

    blockingResponse.requestHeaders = headers;
    return blockingResponse;
};

chrome.webRequest.onBeforeSendHeaders.addListener(handler, requestFilter, extraInfoSpec);

// called from the popup
function setUA(ua, desc) {
    UA = ua;
    description = desc;
}

function clearUA() {
    UA = null;
    description = null;
}

function getUA() {
    return UA;
}

function getDescription() {
    return description;
}
*/