Implement framesDecoded stat in video receive ssrc stats.

Implemented as defined by this pull request: https://github.com/w3c/webrtc-stats/pull/70

BUG=webrtc:6541

Review-Url: https://codereview.webrtc.org/2423823003
Cr-Commit-Position: refs/heads/master@{#14789}
diff --git a/webrtc/api/statstypes.cc b/webrtc/api/statstypes.cc
index 1a9752d..8f25d05 100644
--- a/webrtc/api/statstypes.cc
+++ b/webrtc/api/statstypes.cc
@@ -393,6 +393,8 @@
       return "state";
     case kStatsValueNameDataChannelId:
       return "datachannelid";
+    case kStatsValueNameFramesDecoded:
+      return "framesDecoded";
     case kStatsValueNameFramesEncoded:
       return "framesEncoded";
     case kStatsValueNameCodecImplementationName: