Convert IntelligibilityEnhancer to multi-threaded mode

BUG=581029
R=henrik.lundin@webrtc.org, peah@webrtc.org, turaj@webrtc.org

Review URL: https://codereview.webrtc.org/1766383002 .

Cr-Commit-Position: refs/heads/master@{#11929}
diff --git a/webrtc/modules/audio_processing/noise_suppression_impl.h b/webrtc/modules/audio_processing/noise_suppression_impl.h
index ef30bb1..111e8e1 100644
--- a/webrtc/modules/audio_processing/noise_suppression_impl.h
+++ b/webrtc/modules/audio_processing/noise_suppression_impl.h
@@ -39,6 +39,7 @@
   Level level() const override;
   float speech_probability() const override;
   std::vector<float> NoiseEstimate() override;
+  static size_t num_noise_bins();
 
  private:
   class Suppressor;