Improve stability of the echo detector complexity perf tests.

webrtc/modules/audio_processing/test/performance_timer.h

Index: webrtc/modules/audio_processing/test/performance_timer.h
diff --git a/webrtc/modules/audio_processing/test/performance_timer.h b/webrtc/modules/audio_processing/test/performance_timer.h
index 5921fae07011a16aa8d0a83be8ed92fbb78a4c76..92cda8bd5de4577bbc9de88030f4d112ae466a78 100644
--- a/webrtc/modules/audio_processing/test/performance_timer.h
+++ b/webrtc/modules/audio_processing/test/performance_timer.h
@@ -30,6 +30,11 @@ class PerformanceTimer {
   double GetDurationAverage() const;
   double GetDurationStandardDeviation() const;
 
+  // These methods are the same as those above, but they ignore the first
+  // |number_of_warmup_samples| measurements.
+  double GetDurationAverage(size_t number_of_warmup_samples) const;
+  double GetDurationStandardDeviation(size_t number_of_warmup_samples) const;
+
  private:
   webrtc::Clock* clock_;
   rtc::Optional<int64_t> start_timestamp_us_;