流程：
int H264EncoderImpl::Encode
->int H264EncoderImpl::GetEncodedPartitions
->int32_t VCMEncodedFrameCallback::Encoded
->int32_t ViEEncoder::SendData
->bool PayloadRouter::RoutePayload
->int32_t ModuleRtpRtcpImpl::SendOutgoingData
->int32_t RTPSender::SendOutgoingData
->int32_t RTPSenderVideo::SendVideo
->bool RTPSenderVideo::Send//打包
->int32_t RTPSenderVideo::SendVideoPacket
->int32_t RTPSender::SendToNetwork
->bool PacedSender::SendPacket
将打好的包放入packets_队列，等待发送。
packets_->Push(paced_sender::Packet(
priority, ssrc, sequence_number, capture_time_ms,
clock_->TimeInMilliseconds(), bytes, retransmission, packet_counter_++));
bool ProcessThreadImpl::Process()
int32_t PacedSender::Process()
bool PacedSender::SendPacket
