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

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

Index: webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc
diff --git a/webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc b/webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc
index 31e1d8063600deec1ea6a34eb46b752f695ee520..cd3f00f5d20cba42fd3556714592a4e270ce13cc 100644
--- a/webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc
+++ b/webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc
@@ -1040,6 +1040,10 @@ int VP8EncoderImpl::GetEncodedPartitions(const VideoFrame& input_image,
         // 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) {