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

webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc

 /*
  *  Copyright (c) 2012 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 1022 matching lines @@
         encoded_images_[encoder_idx]._encodedHeight =
             codec_.simulcastStream[stream_idx].height;
         encoded_images_[encoder_idx]._encodedWidth =
             codec_.simulcastStream[stream_idx].width;
         encoded_images_[encoder_idx]
             .adapt_reason_.quality_resolution_downscales =
             quality_scaler_enabled_ ? quality_scaler_.downscale_shift() : -1;
         // Report once per frame (lowest stream always sent).
         encoded_images_[encoder_idx].adapt_reason_.bw_resolutions_disabled =
             (stream_idx == 0) ? bw_resolutions_disabled : -1;
+        int qp_128 = -1;
+        vpx_codec_control(&encoders_[encoder_idx], VP8E_GET_LAST_QUANTIZER,
+                          &qp_128);
+        encoded_images_[encoder_idx].qp_ = qp_128;
         encoded_complete_callback_->Encoded(encoded_images_[encoder_idx],
                                             &codec_specific, &frag_info);
       } else if (codec_.mode == kScreensharing) {
         result = WEBRTC_VIDEO_CODEC_TARGET_BITRATE_OVERSHOOT;
       }
     }
   }
   if (encoders_.size() == 1 && send_stream_[0]) {
     if (encoded_images_[0]._length > 0) {
       int qp;
@@ skipping 350 matching lines @@
     return -1;
   }
   if (vpx_codec_control(copy->decoder_, VP8_SET_REFERENCE, ref_frame_) !=
       VPX_CODEC_OK) {
     return -1;
   }
   return 0;
 }
 
 }  // namespace webrtc