Send runtime settings to the Audio Processing Module (APM)

This CL includes the following changes:
- APM runtime setting (ID + float payload) and unit tests
- Swap queue of APM runtime settings used in AudioProcessingImpl
- runtime settings handler that forwards the settings to APM
  sub-modules when a message is retrieved from the queue
- Unit test placeholder to check that the pre-gain update message
  is correctly delivered

Bug: webrtc:9138
Change-Id: Id22704af15fde2b87a4431f5ce64ad1aeafc5280
Reviewed-on: https://webrtc-review.googlesource.com/69320
Reviewed-by: Per Åhgren <peah@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22873}

modules/audio_processing/audio_processing_impl.h

@@ -66,6 +66,8 @@ class AudioProcessingImpl : public AudioProcessing {
       std::unique_ptr<AudioGenerator> audio_generator) override;
   void DetachPlayoutAudioGenerator() override;
 
+  void SetRuntimeSetting(RuntimeSetting setting) override;
+
   // Capture-side exclusive methods possibly running APM in a
   // multi-threaded manner. Acquire the capture lock.
   int ProcessStream(AudioFrame* frame) override;
@@ -149,6 +151,21 @@ class AudioProcessingImpl : public AudioProcessing {
   std::unique_ptr<ApmDataDumper> data_dumper_;
   static int instance_count_;
 
+  std::unique_ptr<SwapQueue<RuntimeSetting>> runtime_settings_;
+
+  // Class providing thread-safe message pipe functionality for
+  // |runtime_settings_|.
+  class RuntimeSettingEnqueuer {
+   public:
+    explicit RuntimeSettingEnqueuer(
+        SwapQueue<RuntimeSetting>* runtime_settings);
+    ~RuntimeSettingEnqueuer();
+    void Enqueue(RuntimeSetting setting);
+
+   private:
+    SwapQueue<RuntimeSetting>* runtime_settings_;
+  } runtime_settings_enqueuer_;
+
   // Submodule interface implementations.
   std::unique_ptr<HighPassFilter> high_pass_filter_impl_;
 
@@ -239,6 +256,9 @@ class AudioProcessingImpl : public AudioProcessing {
   void InitializePostProcessor() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
   void InitializePreProcessor() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_render_);
 
+  // Handle all the runtime settings in the queue.
+  void HandleRuntimeSettings() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
+
   void EmptyQueuedRenderAudio();
   void AllocateRenderQueue()
       RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_render_, crit_capture_);