Add callbacks for send channel rtp statistics

BUG=2235
R=mflodman@webrtc.org, pbos@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/4449004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5227 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/common_types.h b/common_types.h
index 28099df..55ba552 100644
--- a/common_types.h
+++ b/common_types.h
@@ -235,7 +235,6 @@
 
 // Statistics for an RTCP channel
 struct RtcpStatistics {
- public:
   RtcpStatistics()
     : fraction_lost(0),
       cumulative_lost(0),
@@ -259,19 +258,20 @@
 
 // Data usage statistics for a (rtp) stream
 struct StreamDataCounters {
- public:
   StreamDataCounters()
    : bytes(0),
+     header_bytes(0),
      padding_bytes(0),
      packets(0),
      retransmitted_packets(0),
      fec_packets(0) {}
 
-  uint32_t bytes;
-  uint32_t padding_bytes;
-  uint32_t packets;
-  uint32_t retransmitted_packets;
-  uint32_t fec_packets;
+  uint32_t bytes;  // Payload bytes, excluding RTP headers and padding.
+  uint32_t header_bytes;  // Number of bytes used by RTP headers.
+  uint32_t padding_bytes;  // Number of padding bytes.
+  uint32_t packets;  // Number of packets.
+  uint32_t retransmitted_packets;  // Number of retransmitted packets.
+  uint32_t fec_packets;  // Number of redundancy packets.
 };
 
 // Callback, called whenever byte/packet counts have been updated.
@@ -285,7 +285,6 @@
 
 // Rate statistics for a stream
 struct BitrateStatistics {
- public:
   BitrateStatistics()
     : bitrate_(0),
       packet_rate(0),