Adds AddSamples function to SamplesStatsCounter.

this allows merging two stats counter objects, will be used in a future
CL to merge statistics for multiple video layers.

Bug: webrtc:10365
Change-Id: Iee9c48b68dfd7ba29537c14fc5f4a7c1c333d145
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/131942
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Yves Gerey <yvesg@google.com>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27534}

rtc_base/numerics/samples_stats_counter.h

@@ -31,6 +31,9 @@ class SamplesStatsCounter {
   // Adds sample to the stats in amortized O(1) time.
   void AddSample(double value);
 
+  // Adds samples from another counter.
+  void AddSamples(const SamplesStatsCounter& other);
+
   // Returns if there are any values in O(1) time.
   bool IsEmpty() const { return samples_.empty(); }