Make ExtendedReports::target_bitrate() accessor const

webrtc/modules/rtp_rtcp/source/rtcp_packet/extended_reports.h

 /*
  *  Copyright (c) 2016 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 30 matching lines @@
   void AddDlrrItem(const ReceiveTimeInfo& time_info);
   void SetVoipMetric(const VoipMetric& voip_metric);
   void SetTargetBitrate(const TargetBitrate& target_bitrate);
 
   uint32_t sender_ssrc() const { return sender_ssrc_; }
   const rtc::Optional<Rrtr>& rrtr() const { return rrtr_block_; }
   const Dlrr& dlrr() const { return dlrr_block_; }
   const rtc::Optional<VoipMetric>& voip_metric() const {
     return voip_metric_block_;
   }
-  const rtc::Optional<TargetBitrate>& target_bitrate() {
+  const rtc::Optional<TargetBitrate>& target_bitrate() const {
     return target_bitrate_;
   }
 
  protected:
   bool Create(uint8_t* packet,
               size_t* index,
               size_t max_length,
               RtcpPacket::PacketReadyCallback* callback) const override;
 
  private:
@@ skipping 18 matching lines @@
 
   uint32_t sender_ssrc_;
   rtc::Optional<Rrtr> rrtr_block_;
   Dlrr dlrr_block_;  // Dlrr without items treated same as no dlrr block.
   rtc::Optional<VoipMetric> voip_metric_block_;
   rtc::Optional<TargetBitrate> target_bitrate_;
 };
 }  // namespace rtcp
 }  // namespace webrtc
 #endif  // WEBRTC_MODULES_RTP_RTCP_SOURCE_RTCP_PACKET_EXTENDED_REPORTS_H_