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/histogram.h

@@ -43,6 +43,8 @@ class Histogram {
   // Returns the probability for each bucket in Q30.
   std::vector<int> buckets() const { return buckets_; }
 
+  int forget_factor() const { return base_forget_factor_; }
+
   // Made public for testing.
   static std::vector<int> ScaleBuckets(const std::vector<int>& buckets,
                                        int old_bucket_width,