Move default thresholds from QualityScaler to encoders.

Overriding implementations of VideoEncoder::GetScalingSettings that
want to enable quality scaling must now provide the thresholds.

Bug: webrtc:8830
Change-Id: I75c47cb56ac1b9cf77401684980b3167e485f51c
Reviewed-on: https://webrtc-review.googlesource.com/46622
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22172}

modules/video_coding/codecs/h264/h264_encoder_impl.cc

@@ -31,6 +31,10 @@ namespace {
 
 const bool kOpenH264EncoderDetailedLogging = false;
 
+// QP scaling thresholds.
+static const int kLowH264QpThreshold = 24;
+static const int kHighH264QpThreshold = 37;
+
 // Used by histograms. Values of entries should not be changed.
 enum H264EncoderImplEvent {
   kH264EncoderEventInit = 0,
@@ -503,7 +507,8 @@ int32_t H264EncoderImpl::SetPeriodicKeyFrames(bool enable) {
 }
 
 VideoEncoder::ScalingSettings H264EncoderImpl::GetScalingSettings() const {
-  return VideoEncoder::ScalingSettings(true);
+  return VideoEncoder::ScalingSettings(kLowH264QpThreshold,
+                                       kHighH264QpThreshold);
 }
 
 }  // namespace webrtc