text: var pc = new RTCPeerConnection({});
  
  var onnegotiationneeded = function() {
      console.log(`onnegotiationneeded is triggered.`);
  };
  
  var gotStreamSucc = function(stream) {
      pc.addStream(stream); // 这里会触发 onnegotiationneeded 
  };
  
  var gotStreamErr = function(error) {
      console.error(error);
  };
  
  pc.onnegotiationneeded = onnegotiationneeded;
  
  var conf = {video: true, audio: false};
  navigator.getUserMedia(conf, gotStreamSucc, gotStreamErr);