Connect the stable target rate to the video encoders

The stable target rate is used to make smarter choices in the rate
to chose which layers to enable in SVC or simulcast modes.
the addition of hysteresis, we can improve a call quality by reducing
the amount of resolution switch.


Bug: webrtc:10126
Change-Id: I04d0df9e6bbe247e2f2a668207ff74d475e2464c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150642
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29112}

api/video/video_stream_encoder_interface.h

@@ -85,10 +85,14 @@ class VideoStreamEncoderInterface : public rtc::VideoSinkInterface<VideoFrame> {
 
   // Set the currently estimated network properties. A |target_bitrate|
   // of zero pauses the encoder.
+  // |stable_target_bitrate| is a filtered version of |target_bitrate|. It  is
+  // always less or equal to it. It can be used to avoid rapid changes of
+  // expensive encoding settings, such as resolution.
   // |link_allocation| is the bandwidth available for this video stream on the
   // network link. It is always at least |target_bitrate| but may be higher
   // if we are not network constrained.
   virtual void OnBitrateUpdated(DataRate target_bitrate,
+                                DataRate stable_target_bitrate,
                                 DataRate link_allocation,
                                 uint8_t fraction_lost,
                                 int64_t round_trip_time_ms) = 0;