Move TimestampExtrapolator closer to its single user

The `TimestampExtrapolator` is only used by the `VCMTiming`
class, despite there being references to it from both
`modules/rtp_rtcp/BUILD.gn` and `modules/video_coding/BUILD.gn`.

Bug: webrtc:14111
Change-Id: If1a02a56a0c83b13d619ca08dc76c884fa829369
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/275482
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Johannes Kron <kron@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38093}

modules/video_coding/timing/timestamp_extrapolator.h

@@ -0,0 +1,48 @@
+/*
+ *  Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef MODULES_VIDEO_CODING_TIMING_TIMESTAMP_EXTRAPOLATOR_H_
+#define MODULES_VIDEO_CODING_TIMING_TIMESTAMP_EXTRAPOLATOR_H_
+
+#include <stdint.h>
+
+#include "absl/types/optional.h"
+#include "api/units/timestamp.h"
+#include "modules/include/module_common_types_public.h"
+
+namespace webrtc {
+
+// Not thread safe.
+class TimestampExtrapolator {
+ public:
+  explicit TimestampExtrapolator(Timestamp start);
+  void Update(Timestamp now, uint32_t ts90khz);
+  absl::optional<Timestamp> ExtrapolateLocalTime(uint32_t timestamp90khz) const;
+  void Reset(Timestamp start);
+
+ private:
+  void CheckForWrapArounds(uint32_t ts90khz);
+  bool DelayChangeDetection(double error);
+
+  double w_[2];
+  double p_[2][2];
+  Timestamp start_;
+  Timestamp prev_;
+  absl::optional<int64_t> first_unwrapped_timestamp_;
+  TimestampUnwrapper unwrapper_;
+  absl::optional<int64_t> prev_unwrapped_timestamp_;
+  uint32_t packet_count_;
+  double detector_accumulator_pos_;
+  double detector_accumulator_neg_;
+};
+
+}  // namespace webrtc
+
+#endif  // MODULES_VIDEO_CODING_TIMING_TIMESTAMP_EXTRAPOLATOR_H_