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/PeerConnection/RTCVideoCodec.mm

@@ -54,3 +54,18 @@
 }
 
 @end
+
+@implementation RTCVideoEncoderQpThresholds
+
+@synthesize low = _low;
+@synthesize high = _high;
+
+- (instancetype)initWithThresholdsLow:(NSInteger)low high:(NSInteger)high {
+  if (self = [super init]) {
+    _low = low;
+    _high = high;
+  }
+  return self;
+}
+
+@end

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCVideoCodecH264.mm

@@ -27,6 +27,11 @@ const size_t kDefaultPayloadSize = 1440;
 
 const char kHighProfileExperiment[] = "WebRTC-H264HighProfile";
 
+// These thresholds deviate from the default h264 QP thresholds, as they have been found to work
+// better on devices that support VideoToolbox
+const int kLowH264QpThreshold = 28;
+const int kHighH264QpThreshold = 39;
+
 bool IsHighProfileEnabled() {
   return webrtc::field_trial::IsEnabled(kHighProfileExperiment);
 }
@@ -171,6 +176,11 @@ class H264VideoToolboxDecodeCompleteCallback : public webrtc::DecodedImageCallba
   return _videoToolboxEncoder->SetRates(bitrateKbit, framerate) == WEBRTC_VIDEO_CODEC_OK;
 }
 
+- (RTCVideoEncoderQpThresholds *)scalingSettings {
+  return [[RTCVideoEncoderQpThresholds alloc] initWithThresholdsLow:kLowH264QpThreshold
+                                                               high:kHighH264QpThreshold];
+}
+
 @end
 
 // Decoder.