Allow setting the AGC2 fixed gain during runtime

This CL extends the supported runtime settings in
APM to also comprise the AGC2 fixed gain.
The CL was originally created by Adam Whiteside.

Bug: webrtc:10574
Change-Id: I79b3d6501f1e202b66a9b6018f8a493a56b01f62
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/134101
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27782}
diff --git a/modules/audio_processing/debug.proto b/modules/audio_processing/debug.proto
index 40bd5d5..2af7c81 100644
--- a/modules/audio_processing/debug.proto
+++ b/modules/audio_processing/debug.proto
@@ -83,6 +83,7 @@
 message RuntimeSetting {
   optional float capture_pre_gain = 1;
   optional float custom_render_processing_setting = 2;
+  optional float capture_fixed_post_gain = 3;
 }
 
 message Event {