Add stats for average QP per frame for VP8 (for received video streams).

webrtc/video_frame.h

 /*
  *  Copyright (c) 2014 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 181 matching lines @@
   uint32_t _timeStamp = 0;
   // NTP time of the capture time in local timebase in milliseconds.
   int64_t ntp_time_ms_ = 0;
   int64_t capture_time_ms_ = 0;
   FrameType _frameType = kDeltaFrame;
   uint8_t* _buffer;
   size_t _length;
   size_t _size;
   bool _completeFrame = false;
   AdaptReason adapt_reason_;
+  int qp_ = -1;  // Quantizer value.
 };
 
 }  // namespace webrtc
 #endif  // WEBRTC_VIDEO_FRAME_H_