Add histogram stats for average QP per frame for VP8 (for sent video streams):

webrtc/video/send_statistics_proxy.h

Index: webrtc/video/send_statistics_proxy.h
diff --git a/webrtc/video/send_statistics_proxy.h b/webrtc/video/send_statistics_proxy.h
index 66f03367b2fc4725018b9dcc6a49fb5b9d8a359c..a93ba7965dbc150bb121747b05784a2346b64456 100644
--- a/webrtc/video/send_statistics_proxy.h
+++ b/webrtc/video/send_statistics_proxy.h
@@ -125,6 +125,9 @@ class SendStatisticsProxy : public CpuOveruseMetricsObserver,
     int64_t resolution_update_ms;
     int64_t bitrate_update_ms;
   };
+  struct QpCounters {
+    SampleCounter vp8;
+  };
   void PurgeOldStats() EXCLUSIVE_LOCKS_REQUIRED(crit_);
   VideoSendStream::StreamStats* GetStatsEntry(uint32_t ssrc)
       EXCLUSIVE_LOCKS_REQUIRED(crit_);
@@ -172,6 +175,7 @@ class SendStatisticsProxy : public CpuOveruseMetricsObserver,
     int64_t first_rtp_stats_time_ms_;
     ReportBlockStats report_block_stats_;
     const VideoSendStream::Stats start_stats_;
+    std::map<uint32_t, QpCounters> qp_counters_;  // QP counters mapped by SSRC.
   };
 
   std::unique_ptr<UmaSamplesContainer> uma_container_ GUARDED_BY(crit_);