Add histogram stats for average QP per frame for VP9 (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 108 matching lines @@
     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;
+    SampleCounter vp8;  // QP range: 0-127
+    SampleCounter vp9;  // QP range: 0-255
   };
   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_);
@@ skipping 37 matching lines @@
     const VideoSendStream::Stats start_stats_;
     std::map<int, QpCounters>
         qp_counters_;  // QP counters mapped by spatial idx.
   };
 
   std::unique_ptr<UmaSamplesContainer> uma_container_ GUARDED_BY(crit_);
 };
 
 }  // namespace webrtc
 #endif  // WEBRTC_VIDEO_SEND_STATISTICS_PROXY_H_