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/mock/mock_delay_manager.h

@@ -23,12 +23,16 @@ class MockDelayManager : public DelayManager {
  public:
   MockDelayManager(size_t max_packets_in_buffer,
                    int base_min_target_delay_ms,
+                   int histogram_quantile,
+                   HistogramMode histogram_mode,
                    bool enable_rtx_handling,
                    DelayPeakDetector* peak_detector,
                    const TickTimer* tick_timer,
                    std::unique_ptr<Histogram> histogram)
       : DelayManager(max_packets_in_buffer,
                      base_min_target_delay_ms,
+                     histogram_quantile,
+                     histogram_mode,
                      enable_rtx_handling,
                      peak_detector,
                      tick_timer,