Activate the pre-amplifier in AudioProcessing.

It's a module for applying a gain to the capture signal.
The gain is the first processing step in APM.

After this CL, these two features work:
* The PreAmplifier can be activated with
  AudioProcessing::Config::pre_amplifier
* The PreApmlifier can be controlled after APM creation by
  AudioProcessing::SetRuntimeSetting.

What's left is a change to AecDumps and to AecDump-replay.

NOTRY=True # 1-line change, tests just passed.

Bug: webrtc:9138
Change-Id: I85b3af511695b0a9cec2eed6fee7f05080305e1d
Reviewed-on: https://webrtc-review.googlesource.com/69811
Commit-Queue: Alex Loiko <aleloi@webrtc.org>
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22881}
diff --git a/modules/audio_processing/audio_processing_impl.h b/modules/audio_processing/audio_processing_impl.h
index 7f9cbcd..7da6e75 100644
--- a/modules/audio_processing/audio_processing_impl.h
+++ b/modules/audio_processing/audio_processing_impl.h
@@ -186,6 +186,7 @@
                 bool beamformer_enabled,
                 bool adaptive_gain_controller_enabled,
                 bool gain_controller2_enabled,
+                bool pre_amplifier_enabled,
                 bool echo_controller_enabled,
                 bool voice_activity_detector_enabled,
                 bool level_estimator_enabled,
@@ -209,6 +210,7 @@
     bool beamformer_enabled_ = false;
     bool adaptive_gain_controller_enabled_ = false;
     bool gain_controller2_enabled_ = false;
+    bool pre_amplifier_enabled_ = false;
     bool echo_controller_enabled_ = false;
     bool level_estimator_enabled_ = false;
     bool voice_activity_detector_enabled_ = false;
@@ -253,6 +255,7 @@
   void InitializeLowCutFilter() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
   void InitializeEchoController() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
   void InitializeGainController2() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
+  void InitializePreAmplifier() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
   void InitializePostProcessor() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_capture_);
   void InitializePreProcessor() RTC_EXCLUSIVE_LOCKS_REQUIRED(crit_render_);