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/statscollector.cc b/webrtc/api/statscollector.cc
index fab33cd..819f31b 100644
--- a/webrtc/api/statscollector.cc
+++ b/webrtc/api/statscollector.cc
@@ -234,6 +234,7 @@
     { StatsReport::kStatsValueNamePlisSent, info.plis_sent },
     { StatsReport::kStatsValueNameRenderDelayMs, info.render_delay_ms },
     { StatsReport::kStatsValueNameTargetDelayMs, info.target_delay_ms },
+    { StatsReport::kStatsValueNameFramesDecoded, info.frames_decoded },
   };
 
   for (const auto& i : ints)