How to reach 60 FPS when recording screen in Chrome?

Hi guys, I'm trying to record a screen in a Chrome extension, and for this I use

 navigator.webkitGetUserMedia(videoConstraints, function(stream) { ...

as videoConstraints I posted:

var videoConstraints = {
    audio: false,
    video: {
    mandatory: { chromeMediaSource: 'screen', maxWidth: 960,
            maxHeight: 720, minWidth:960, minHeight:720  },
    optional: [
      { minFrameRate: 60 },
      { maxWidth: 640 },
      { maxHeigth: 480 }
    ]
}
};

Unfortunately, the result is rather backward. Is there a way that I can achieve 60 FPS with this method? Or should I look for other options? Can NaCl Become a Way?

thank