ObjC: Fix quality scaling for injected encoders

We missed to implement quality scaling in the original CL
https://codereview.webrtc.org/2977213002/. This CL implements it.

Note that the ObjC interface for scalingSettings is slightly different from the C++
interface in that we require explicit QP thresholds to turn quality scaling on, i.e.
we don't provide default values. I think this is more modular as we want to move
codec specific knowledge out from the WebRTC core. I would like to update the
C++ webrtc::VideoEncoder interface to do the same in another CL.

BUG=webrtc:7924

Review-Url: https://codereview.webrtc.org/2991123002
Cr-Commit-Position: refs/heads/master@{#19202}

webrtc/sdk/objc/Framework/Classes/VideoToolbox/encoder.mm

@@ -757,6 +757,8 @@ void H264VideoToolboxEncoder::OnEncodedFrame(
   bitrate_adjuster_.Update(frame._length);
 }
 
+// TODO(magjed): This function is not used by RTCVideoEncoderH264, but this whole file will be
+// removed soon and inlined as ObjC.
 VideoEncoder::ScalingSettings H264VideoToolboxEncoder::GetScalingSettings()
     const {
   return VideoEncoder::ScalingSettings(true, internal::kLowH264QpThreshold,

webrtc/sdk/objc/Framework/Classes/VideoToolbox/objc_video_encoder_factory.mm

@@ -111,6 +111,13 @@ class ObjCVideoEncoder : public VideoEncoder {
 
   bool SupportsNativeHandle() const { return true; }
 
+  VideoEncoder::ScalingSettings GetScalingSettings() const {
+    RTCVideoEncoderQpThresholds* qp_thresholds = [encoder_ scalingSettings];
+    return qp_thresholds ?
+        ScalingSettings(true /* enabled */, qp_thresholds.low, qp_thresholds.high) :
+        ScalingSettings(false /* enabled */);
+  }
+
  private:
   id<RTCVideoEncoder> encoder_;
 };