Make GetConfig() part of the AudioProcessing interface

BUG=chromium:708475
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

Review-Url: https://codereview.webrtc.org/2802603003
Cr-Commit-Position: refs/heads/master@{#17540}
diff --git a/webrtc/modules/audio_processing/audio_processing_impl.h b/webrtc/modules/audio_processing/audio_processing_impl.h
index 01b640f..7993149 100644
--- a/webrtc/modules/audio_processing/audio_processing_impl.h
+++ b/webrtc/modules/audio_processing/audio_processing_impl.h
@@ -134,9 +134,9 @@
   NoiseSuppression* noise_suppression() const override;
   VoiceDetection* voice_detection() const override;
 
-  // TODO(peah): Remove these two methods once the new API allows that.
+  // TODO(peah): Remove MutateConfig once the new API allows that.
   void MutateConfig(rtc::FunctionView<void(AudioProcessing::Config*)> mutator);
-  AudioProcessing::Config GetConfig() const;
+  AudioProcessing::Config GetConfig() const override;
 
  protected:
   // Overridden in a mock.