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/Headers/WebRTC/RTCVideoCodec.h

@@ -104,6 +104,17 @@ RTC_EXPORT
 
 @end
 
+/** QP thresholds for encoder. Corresponds to webrtc::VideoEncoder::QpThresholds. */
+RTC_EXPORT
+@interface RTCVideoEncoderQpThresholds : NSObject
+
+- (instancetype)initWithThresholdsLow:(NSInteger)low high:(NSInteger)high;
+
+@property(nonatomic, readonly) NSInteger low;
+@property(nonatomic, readonly) NSInteger high;
+
+@end
+
 /** Protocol for encoder implementations. */
 RTC_EXPORT
 @protocol RTCVideoEncoder <NSObject>
@@ -118,6 +129,13 @@ RTC_EXPORT
            frameTypes:(NSArray<NSNumber *> *)frameTypes;
 - (BOOL)setBitrate:(uint32_t)bitrateKbit framerate:(uint32_t)framerate;
 
+/** Returns QP scaling settings for encoder. The quality scaler adjusts the resolution in order to
+ *  keep the QP from the encoded images within the given range. Returning nil from this function
+ *  disables quality scaling. */
+- (RTCVideoEncoderQpThresholds *)scalingSettings;
+
+// TODO(andersc): Add implementationName method.
+
 @end
 
 /** Protocol for decoder implementations. */
@@ -135,6 +153,8 @@ RTC_EXPORT
       codecSpecificInfo:(__nullable id<RTCCodecSpecificInfo>)info
            renderTimeMs:(int64_t)renderTimeMs;
 
+// TODO(andersc): Add implementationName method.
+
 @end
 
 NS_ASSUME_NONNULL_END