Refactor delay manager.

Split out `RelativeArrivalDelayTracker` and `DelayOptimizer` logic.

This is in preparation for adding another `DelayOptimizer` specialized in handling reordered packets.

Bug: webrtc:10178
Change-Id: Id3c1746d91980b171fa524f9b2b71cf11fc75f64
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/231224
Commit-Queue: Jakob Ivarsson <jakobi@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34938}

modules/audio_coding/neteq/underrun_optimizer_unittest.cc

@@ -0,0 +1,42 @@
+/*
+ *  Copyright (c) 2021 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.
+ */
+
+#include "modules/audio_coding/neteq/underrun_optimizer.h"
+
+#include "test/gtest.h"
+
+namespace webrtc {
+
+namespace {
+
+constexpr int kDefaultHistogramQuantile = 1020054733;  // 0.95 in Q30.
+constexpr int kForgetFactor = 32745;                   // 0.9993 in Q15.
+
+}  // namespace
+
+TEST(UnderrunOptimizerTest, ResamplePacketDelays) {
+  TickTimer tick_timer;
+  constexpr int kResampleIntervalMs = 500;
+  UnderrunOptimizer underrun_optimizer(&tick_timer, kDefaultHistogramQuantile,
+                                       kForgetFactor, absl::nullopt,
+                                       kResampleIntervalMs);
+
+  // The histogram should be updated once with the maximum delay observed for
+  // the following sequence of updates.
+  for (int i = 0; i < 500; i += 20) {
+    underrun_optimizer.Update(i);
+    EXPECT_FALSE(underrun_optimizer.GetOptimalDelayMs());
+  }
+  tick_timer.Increment(kResampleIntervalMs / tick_timer.ms_per_tick() + 1);
+  underrun_optimizer.Update(0);
+  EXPECT_EQ(underrun_optimizer.GetOptimalDelayMs(), 500);
+}
+
+}  // namespace webrtc