Preserve level controller output when no other effects are active

This CL ensures that the output of the level controller is kept
when no other submodules in APM are active

BUG=webrtc:7697,

Review-Url: https://codereview.webrtc.org/2902723002
Cr-Commit-Position: refs/heads/master@{#18230}
diff --git a/webrtc/modules/audio_processing/audio_processing_impl.h b/webrtc/modules/audio_processing/audio_processing_impl.h
index 7fc81f8..0985257 100644
--- a/webrtc/modules/audio_processing/audio_processing_impl.h
+++ b/webrtc/modules/audio_processing/audio_processing_impl.h
@@ -173,6 +173,7 @@
                 bool transient_suppressor_enabled);
     bool CaptureMultiBandSubModulesActive() const;
     bool CaptureMultiBandProcessingActive() const;
+    bool CaptureFullBandProcessingActive() const;
     bool RenderMultiBandSubModulesActive() const;
     bool RenderMultiBandProcessingActive() const;