WebRTC如何通過參數控制編碼模式切換（DIsabled模式）

bool WebRtcVideoChannel::AddSendStream(const StreamParams& sp) 

調用AddSendStream（）方法時，首先檢驗StreamParams參數是否有效，然後判斷SSRC是否可用。創建WebRtcVideoSendStream實例的時候，將參數傳遞給WebRtcVideoSendStream對象。參數sp經過CreateRtpParametersWithEncodings處理後，賦值給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