Use bitstream-level QP for libvpx VP8 quality.

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 b20ce4921c0227f630a41283e78ebc0af8876456..c7fa66eae70d46e91588cf3fb722bfd3b6c041e7 100644
--- a/webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc
+++ b/webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc
@@ -600,8 +600,8 @@ int VP8EncoderImpl::InitEncode(const VideoCodec* inst,
   rps_.Init();
   // Disable both high-QP limits and framedropping. Both are handled by libvpx
   // internally.
-  const int kLowQpThreshold = 18;
-  const int kDisabledBadQpThreshold = 64;
+  const int kLowQpThreshold = 23;

[stefan-webrtc, 2016/04/15 09:21:08]

Please clarify in the code why this was chosen to be 23.

+  const int kDisabledBadQpThreshold = 128;
   // TODO(glaznev/sprang): consider passing codec initial bitrate to quality
   // scaler to avoid starting with HD for low initial bitrates.
   quality_scaler_.Init(kLowQpThreshold, kDisabledBadQpThreshold, false, 0, 0, 0,
@@ -1053,9 +1053,9 @@ int VP8EncoderImpl::GetEncodedPartitions(const VideoFrame& input_image,
   }
   if (encoders_.size() == 1 && send_stream_[0]) {
     if (encoded_images_[0]._length > 0) {
-      int qp;
-      vpx_codec_control(&encoders_[0], VP8E_GET_LAST_QUANTIZER_64, &qp);
-      quality_scaler_.ReportQP(qp);
+      int qp_128;
+      vpx_codec_control(&encoders_[0], VP8E_GET_LAST_QUANTIZER, &qp_128);
+      quality_scaler_.ReportQP(qp_128);
     } else {
       quality_scaler_.ReportDroppedFrame();
     }