Make ExtendedReports::target_bitrate() accessor const

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

Index: webrtc/modules/rtp_rtcp/source/rtcp_packet/extended_reports.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_packet/extended_reports.h b/webrtc/modules/rtp_rtcp/source/rtcp_packet/extended_reports.h
index 21fadd833814646bf8d81aed946edfd94ac893c4..09723ef949137ff2a175e70c6713e665c50dd6cb 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_packet/extended_reports.h
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_packet/extended_reports.h
@@ -48,7 +48,7 @@ class ExtendedReports : public RtcpPacket {
   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_;
   }