Add protection for RTCPSender::max_packet_size_.

webrtc/modules/rtp_rtcp/source/rtcp_sender.h

Index: webrtc/modules/rtp_rtcp/source/rtcp_sender.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_sender.h b/webrtc/modules/rtp_rtcp/source/rtcp_sender.h
index ccfbd4812491316892f4a58d5fbfd5326f437add..76abf63853d03fd4657bf893dfb3f7361c63f588 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_sender.h
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_sender.h
@@ -238,7 +238,7 @@ class RTCPSender {
       GUARDED_BY(critical_section_rtcp_sender_);
   uint32_t tmmbr_send_bps_ GUARDED_BY(critical_section_rtcp_sender_);
   uint32_t packet_oh_send_ GUARDED_BY(critical_section_rtcp_sender_);
-  size_t max_packet_size_;
+  size_t max_packet_size_ GUARDED_BY(critical_section_rtcp_sender_);
 
   // APP
   uint8_t app_sub_type_ GUARDED_BY(critical_section_rtcp_sender_);