Add overhead per packet observer to the rtp_sender.

webrtc/modules/rtp_rtcp/include/rtp_rtcp.h

Index: webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
diff --git a/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h b/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
index 830d895774d2559ece632a707c7b73ee91949b4f..2812ad989060cc16eba30e095cb314672c123203 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
@@ -27,6 +27,7 @@
 namespace webrtc {
 
 // Forward declarations.
+class OverheadObserver;
 class RateLimiter;
 class ReceiveStatistics;
 class RemoteBitrateEstimator;
@@ -89,6 +90,7 @@ class RtpRtcp : public Module {
     RtcEventLog* event_log = nullptr;
     SendPacketObserver* send_packet_observer = nullptr;
     RateLimiter* retransmission_rate_limiter = nullptr;
+    OverheadObserver* overhead_observer = nullptr;
 
    private:
     RTC_DISALLOW_COPY_AND_ASSIGN(Configuration);