title: webrtc - video get blob, but it remain black
body: <p>I run my webrtc code with chrome 21.</p>
  
  <p>If I open two tabs in the same chrome, and then open page with webrtc code inside.
  One tab is for sending video stream; one tab is for receiving video stream 
  It works fine.</p>
  
  <p>However, If I open page with two Incognito mode or two different chrome browser,
  I can get the <strong>sdp</strong> and <strong>candidate</strong> information correctly. It seems that video can decode the information.</p>
  
  <p>In remote video, I can see only <img src="https://i.stack.imgur.com/IZU9O.png" alt="enter image description here"></p>
  
  <p>Besides, it seems crash. I tried to click "close chrome" but useless. </p>
  
  <p>Does anyone have any similar problem?</p>