DataChannel[Interface]::[message/bytes]_[sent/received]() added.

These are required for the RTCDataChannelStats[1] that will be collected
in a follow-up CL.

[1] https://w3c.github.io/webrtc-stats/#dcstats-dict*

BUG=chromium:654927, chromium:627816

Review-Url: https://codereview.webrtc.org/2413803002
Cr-Commit-Position: refs/heads/master@{#14616}
diff --git a/webrtc/api/datachannelinterface.h b/webrtc/api/datachannelinterface.h
index 53c11d4..8b788c0 100644
--- a/webrtc/api/datachannelinterface.h
+++ b/webrtc/api/datachannelinterface.h
@@ -125,6 +125,10 @@
 
   virtual int id() const = 0;
   virtual DataState state() const = 0;
+  virtual uint32_t messages_sent() const = 0;
+  virtual uint64_t bytes_sent() const = 0;
+  virtual uint32_t messages_received() const = 0;
+  virtual uint64_t bytes_received() const = 0;
   // The buffered_amount returns the number of bytes of application data
   // (UTF-8 text and binary data) that have been queued using SendBuffer but
   // have not yet been transmitted to the network.