Enable congestion window pushback to reduce bitrate by only drop video frames.

With current congestion window pushback, when congestion window is filling up, it will reduce bitrate directly and encoder may reduce encode quality, resolution, or framerate to adapt to the allocated bitrate, the behavior is depending on the degradation preference.
This change enable congestion window to only drop frames to reduce bitrate (when needed) instead of reduce general bitrate allocation.

Bug: webrtc:11334
Change-Id: I9cf5c20a0858c4d07d006942abe72aa5e1f7cb38
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168059
Commit-Queue: Ying Wang <yinwa@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30483}

api/video/video_stream_encoder_interface.h

@@ -95,7 +95,8 @@ class VideoStreamEncoderInterface : public rtc::VideoSinkInterface<VideoFrame> {
                                 DataRate stable_target_bitrate,
                                 DataRate link_allocation,
                                 uint8_t fraction_lost,
-                                int64_t round_trip_time_ms) = 0;
+                                int64_t round_trip_time_ms,
+                                double cwnd_reduce_ratio) = 0;
 
   // Register observer for the bitrate allocation between the temporal
   // and spatial layers.