Add target bitrate headroom signal to VideoStreamEncoder.

This CL plumbs an additional signal from VideoSendStream down to
VideoStreamEncoder, namely the amount of headroom that's left between
the encoder max bitrate and the current bitrate allocation for the
media track.

This will be used in follow-up CLs to tune encoder rate adjustment
and some codec specific paramaters a bit differently, based on the
knowledge if we are network constrained or not.

Bug: webrtc:10155
Change-Id: Ic6ccc79be5c6845468bab65b4ca9918b56923fa4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/125981
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27008}

api/video/video_stream_encoder_interface.h

@@ -14,6 +14,7 @@
 #include <vector>
 
 #include "api/rtp_parameters.h"  // For DegradationPreference.
+#include "api/units/data_rate.h"
 #include "api/video/video_bitrate_allocator.h"
 #include "api/video/video_sink_interface.h"
 #include "api/video/video_source_interface.h"
@@ -79,7 +80,8 @@ class VideoStreamEncoderInterface : public rtc::VideoSinkInterface<VideoFrame> {
 
   // Set the currently estimated network properties. A |bitrate_bps|
   // of zero pauses the encoder.
-  virtual void OnBitrateUpdated(uint32_t bitrate_bps,
+  virtual void OnBitrateUpdated(DataRate target_bitrate,
+                                DataRate target_headroom,
                                 uint8_t fraction_lost,
                                 int64_t round_trip_time_ms) = 0;