Cleanup RtpToNtpEstimator

- Use NtpTime instead of pair of uint32_t to represent ntp time
- Increase precision estimate with NtpTime precision instead of ms precision
- Hide helper structs as private types
- Modernize interface to prefer return values over output parameters
- embed LinearRegression helper into the only user: UpdateParameters

Bug: webrtc:13757
Change-Id: I0a62a03e2869b2ae1eacaa15253accc43ba0a598
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/254780
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36232}

video/stream_synchronization.cc

@@ -35,19 +35,19 @@ bool StreamSynchronization::ComputeRelativeDelay(
     const Measurements& audio_measurement,
     const Measurements& video_measurement,
     int* relative_delay_ms) {
-  int64_t audio_last_capture_time_ms;
-  if (!audio_measurement.rtp_to_ntp.Estimate(audio_measurement.latest_timestamp,
-                                             &audio_last_capture_time_ms)) {
+  NtpTime audio_last_capture_time =
+      audio_measurement.rtp_to_ntp.Estimate(audio_measurement.latest_timestamp);
+  if (!audio_last_capture_time.Valid()) {
     return false;
   }
-  int64_t video_last_capture_time_ms;
-  if (!video_measurement.rtp_to_ntp.Estimate(video_measurement.latest_timestamp,
-                                             &video_last_capture_time_ms)) {
-    return false;
-  }
-  if (video_last_capture_time_ms < 0) {
+  NtpTime video_last_capture_time =
+      video_measurement.rtp_to_ntp.Estimate(video_measurement.latest_timestamp);
+  if (!video_last_capture_time.Valid()) {
     return false;
   }
+  int64_t audio_last_capture_time_ms = audio_last_capture_time.ToMs();
+  int64_t video_last_capture_time_ms = video_last_capture_time.ToMs();
+
   // Positive diff means that video_measurement is behind audio_measurement.
   *relative_delay_ms =
       video_measurement.latest_receive_time_ms -