Add RELATIVE_ARRIVAL_DELAY histogram mode to DelayManager.

- This mode estimates relative packet arrival delay for each incoming packet and adds that value to the histogram.
- The histogram buckets are 20 milliseconds each instead of whole packets.
- The functionality is enabled with a field trial for experimentation.

Bug: webrtc:10333
Change-Id: I8f7499c56802fc1aa1ced2f5310fdd2ef1403515
Reviewed-on: https://webrtc-review.googlesource.com/c/123923
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26871}

modules/audio_coding/neteq/neteq_impl_unittest.cc

@@ -97,7 +97,8 @@ class NetEqImplTest : public ::testing::Test {
 
     if (use_mock_delay_manager_) {
       std::unique_ptr<MockDelayManager> mock(new MockDelayManager(
-          config_.max_packets_in_buffer, config_.min_delay_ms,
+          config_.max_packets_in_buffer, config_.min_delay_ms, 1020054733,
+          DelayManager::HistogramMode::INTER_ARRIVAL_TIME,
           config_.enable_rtx_handling, delay_peak_detector_, tick_timer_,
           absl::make_unique<Histogram>(50, 32745)));
       mock_delay_manager_ = mock.get();