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