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/include/audio_processing.h

@@ -302,6 +302,32 @@ class AudioProcessing : public rtc::RefCountInterface {
     kStereoAndKeyboard
   };
 
+  // Specifies the properties of a setting to be passed to AudioProcessing at
+  // runtime.
+  class RuntimeSetting {
+   public:
+    enum class Type { kNotSpecified, kCapturePreGain };
+
+    RuntimeSetting() : type_(Type::kNotSpecified), value_(0.f) {}
+    ~RuntimeSetting() = default;
+
+    static RuntimeSetting CreateCapturePreGain(float gain) {
+      RTC_DCHECK_GE(gain, 1.f) << "Attenuation is not allowed.";
+      return {Type::kCapturePreGain, gain};
+    }
+
+    Type type() const { return type_; }
+    void GetFloat(float* value) const {
+      RTC_DCHECK(value);
+      *value = value_;
+    }
+
+   private:
+    RuntimeSetting(Type id, float value) : type_(id), value_(value) {}
+    Type type_;
+    float value_;
+  };
+
   ~AudioProcessing() override {}
 
   // Initializes internal states, while retaining all user settings. This
@@ -359,6 +385,9 @@ class AudioProcessing : public rtc::RefCountInterface {
   // Default false.
   virtual void set_output_will_be_muted(bool muted) = 0;
 
+  // Enqueue a runtime setting.
+  virtual void SetRuntimeSetting(RuntimeSetting setting) = 0;
+
   // Processes a 10 ms |frame| of the primary audio stream. On the client-side,
   // this is the near-end (or captured) audio.
   //