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 1e4da64220d1d91577a38ed3e1277a349bcdff59..fdcdeef041b9f9fabe4559a36c93232560c4780d 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
@@ -25,6 +25,7 @@
 namespace webrtc {
 
 // Forward declarations.
+class OverheadSizeObserver;
 class RateLimiter;
 class ReceiveStatistics;
 class RemoteBitrateEstimator;
@@ -83,6 +84,7 @@ class RtpRtcp : public Module {
     RtcEventLog* event_log = nullptr;
     SendPacketObserver* send_packet_observer = nullptr;
     RateLimiter* retransmission_rate_limiter = nullptr;
+    OverheadSizeObserver* overhead_size_observer = nullptr;
 
    private:
     RTC_DISALLOW_COPY_AND_ASSIGN(Configuration);