WebRTC如何通过参数控制编码模式切换（DIsabled模式）

bool WebRtcVideoChannel::AddSendStream(const StreamParams& sp) 

调用AddSendStream（）方法时，首先检验StreamParams参数是否有效，然后判断SSRC是否可用。创建WebRtcVideoSendStream实例的时候，将参数传递给WebRtcVideoSendStream对象。参数sp经过CreateRtpParametersWithEncodings处理后，赋值给rtp_parameters_。

webrtc::DegradationPreference
WebRtcVideoChannel::WebRtcVideoSendStream::GetDegradationPreference() const {
  // Do not adapt resolution for screen content as this will likely
  // result in blurry and unreadable text.
  // |this| acts like a VideoSource to make sure SinkWants are handled on the
  // correct thread.
  webrtc::DegradationPreference degradation_preference;
  if (rtp_parameters_.degradation_preference !=
      webrtc::DegradationPreference::BALANCED) {
    // If the degradationPreference is different from the default value, assume
    // it is what we want, regardless of trials or other internal settings.
    degradation_preference = rtp_parameters_.degradation_preference;
  } else