Add overhead per packet observer to the rtp_sender.

webrtc/common_types.h

Index: webrtc/common_types.h
diff --git a/webrtc/common_types.h b/webrtc/common_types.h
index 24ba00a8450e4e9869c544f772c774105af0100c..89361d499874c6b36fc6a1ca863fd80ee42edf48 100644
--- a/webrtc/common_types.h
+++ b/webrtc/common_types.h
@@ -298,6 +298,14 @@ class SendPacketObserver {
                             uint32_t ssrc) = 0;
 };
 
+// Callback, used to notify an observer when the overhead per packet
+// has changed.
+class OverheadSizeObserver {
+ public:
+  virtual ~OverheadSizeObserver() = default;
+  virtual void OnOverheadSizeChanged(int overhead_bytes_per_packet) = 0;
+};
+
 // ==================================================================
 // Voice specific types
 // ==================================================================