DataChannelInterface default impl of [messages/bytes]_[sent/received].

The default implementations are provided as to not break Chromium mocks,
as soon as we have done a successful roll they should be updated and the
default implementations removed.

TBR=hta@webrtc.org, deadbeef@webrtc.org
NOTRY=True
BUG=chromium:654927

Review-Url: https://codereview.webrtc.org/2414613003
Cr-Commit-Position: refs/heads/master@{#14617}
diff --git a/webrtc/api/datachannelinterface.h b/webrtc/api/datachannelinterface.h
index 8b788c0..ddeb1e5 100644
--- a/webrtc/api/datachannelinterface.h
+++ b/webrtc/api/datachannelinterface.h
@@ -125,10 +125,13 @@
 
   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;
+  // TODO(hbos): Default implementations of [messages/bytes]_[sent/received] as
+  // to not break Chromium. Fix Chromium's implementation as soon as this rolls.
+  // crbug.com/654927
+  virtual uint32_t messages_sent() const { return 0; }
+  virtual uint64_t bytes_sent() const { return 0; }
+  virtual uint32_t messages_received() const { return 0; }
+  virtual uint64_t bytes_received() const { return 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.