Expose codec implementation names in stats.

Used to distinguish between software/hardware encoders/decoders and
other implementation differences. Useful for tracking quality
regressions related to specific implementations.

BUG=webrtc:4897
R=hta@webrtc.org, mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1406903002 .

Cr-Commit-Position: refs/heads/master@{#11084}
diff --git a/talk/media/base/mediachannel.h b/talk/media/base/mediachannel.h
index 0a59019..f6fb77d 100644
--- a/talk/media/base/mediachannel.h
+++ b/talk/media/base/mediachannel.h
@@ -783,6 +783,7 @@
   }
 
   std::vector<SsrcGroup> ssrc_groups;
+  std::string encoder_implementation_name;
   int packets_cached;
   int firs_rcvd;
   int plis_rcvd;
@@ -828,6 +829,7 @@
   }
 
   std::vector<SsrcGroup> ssrc_groups;
+  std::string decoder_implementation_name;
   int packets_concealed;
   int firs_sent;
   int plis_sent;