Make OverheadObserver::OnOverheadChanged count RTP headers only

webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 319 matching lines @@
     if (flexfec_enabled())
       flexfec_sender_->SetFecParameters(fec_params);
     if (ulpfec_enabled())
       ulpfec_generator_.SetFecParameters(fec_params);
 
     fec_packet_overhead = CalculateFecPacketOverhead();
     red_enabled = this->red_enabled();
     retransmission_settings = retransmission_settings_;
   }
 
-  size_t packet_capacity = rtp_sender_->MaxPayloadLength() -
+  size_t packet_capacity = rtp_sender_->MaxRtpPacketSize() -
                            fec_packet_overhead -
                            (rtp_sender_->RtxStatus() ? kRtxHeaderSize : 0);
   RTC_DCHECK_LE(packet_capacity, rtp_header->capacity());
   RTC_DCHECK_GT(packet_capacity, rtp_header->headers_size());
   size_t max_data_payload_length = packet_capacity - rtp_header->headers_size();
 
   std::unique_ptr<RtpPacketizer> packetizer(RtpPacketizer::Create(
       video_type, max_data_payload_length,
       video_header ? &(video_header->codecHeader) : nullptr, frame_type));
   // Media packet storage.
@@ skipping 64 matching lines @@
   rtc::CritScope cs(&crit_);
   return retransmission_settings_;
 }
 
 void RTPSenderVideo::SetSelectiveRetransmissions(uint8_t settings) {
   rtc::CritScope cs(&crit_);
   retransmission_settings_ = settings;
 }
 
 }  // namespace webrtc