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

webrtc/video/send_statistics_proxy.h

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 107 matching lines @@
    private:
     int Fraction(int min_required_samples, float multiplier) const;
     int sum;
     int num_samples;
   };
   struct StatsUpdateTimes {
     StatsUpdateTimes() : resolution_update_ms(0), bitrate_update_ms(0) {}
     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_);
 
   Clock* const clock_;
   const VideoSendStream::Config config_;
   rtc::CriticalSection crit_;
   VideoEncoderConfig::ContentType content_type_ GUARDED_BY(crit_);
   VideoSendStream::Stats stats_ GUARDED_BY(crit_);
   uint32_t last_sent_frame_timestamp_ GUARDED_BY(crit_);
@@ skipping 27 matching lines @@
     BoolSampleCounter bw_limited_frame_counter_;
     SampleCounter bw_resolutions_disabled_counter_;
     SampleCounter delay_counter_;
     SampleCounter max_delay_counter_;
     rtc::RateTracker input_frame_rate_tracker_;
     rtc::RateTracker sent_frame_rate_tracker_;
     int64_t first_rtcp_stats_time_ms_;
     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_);
 };
 
 }  // namespace webrtc
 #endif  // WEBRTC_VIDEO_SEND_STATISTICS_PROXY_H_