Add histogram stats for send delay for a sent video stream.

webrtc/common_types.h

Index: webrtc/common_types.h
diff --git a/webrtc/common_types.h b/webrtc/common_types.h
index e171e1bd8af52e7336f0447d3bac3fadf03a2719..ef65e93d00058329e3bc3648b5d8410b9db2c836 100644
--- a/webrtc/common_types.h
+++ b/webrtc/common_types.h
@@ -296,6 +296,18 @@ class SendSideDelayObserver {
                                     uint32_t ssrc) = 0;
 };
 
+// Callback, used to notify an observer whenever a packet is sent to the
+// transport.
+// TODO(asapersson): This class will remove the need for SendSideDelayObserver.
+// Remove SendSideDelayObserver once possible.
+class SendPacketObserver {
+ public:
+  virtual ~SendPacketObserver() {}
+  virtual void OnSendPacket(uint16_t packet_id,
+                            int64_t capture_time_ms,
+                            uint32_t ssrc) = 0;
+};
+
 // ==================================================================
 // Voice specific types
 // ==================================================================