APM pre-gain sub-module: code improvements.

- No need to have a unique ptr for the swap queue
- Remove default case from the switch in
  AudioProcessingImpl::HandleRuntimeSettings()

Bug: webrtc:9138
Change-Id: I346ba1db6510b5caa637510298b67ead07197b81
Reviewed-on: https://webrtc-review.googlesource.com/71164
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22958}
diff --git a/modules/audio_processing/audio_processing_impl.h b/modules/audio_processing/audio_processing_impl.h
index ff563b8..a49924d 100644
--- a/modules/audio_processing/audio_processing_impl.h
+++ b/modules/audio_processing/audio_processing_impl.h
@@ -149,7 +149,7 @@
   std::unique_ptr<ApmDataDumper> data_dumper_;
   static int instance_count_;
 
-  std::unique_ptr<SwapQueue<RuntimeSetting>> runtime_settings_;
+  SwapQueue<RuntimeSetting> runtime_settings_;
 
   // Class providing thread-safe message pipe functionality for
   // |runtime_settings_|.
@@ -161,7 +161,7 @@
     void Enqueue(RuntimeSetting setting);
 
    private:
-    SwapQueue<RuntimeSetting>* runtime_settings_;
+    SwapQueue<RuntimeSetting>& runtime_settings_;
   } runtime_settings_enqueuer_;
 
   // Submodule interface implementations.