Make the extra seturation margin configurable.

The extra saturation margin is a setting for the SaturationProtector
in GainController2. The higher it is, the less gain GC2 will apply. In
this CL we pipe the setting up to audio_processing.h. Now the setting
can be set at a high level.

Also in this CL add a few (missing, they should have been there
already) tests for the GC2 and GC2 with saturation margin.

Bug: webrtc:7494
Change-Id: I1b61f1662e6c6a8817fd5b0e845339694bf8d50d
Reviewed-on: https://webrtc-review.googlesource.com/c/109001
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Alex Loiko <aleloi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25470}

modules/audio_processing/include/audio_processing.h

@@ -273,6 +273,7 @@ class AudioProcessing : public rtc::RefCountInterface {
     struct GainController2 {
       bool enabled = false;
       bool adaptive_digital_mode = true;
+      float extra_saturation_margin_db = 2.f;
       float fixed_gain_db = 0.f;
     } gain_controller2;