First try on a fake analog volume control interface for use in audioproc_f.

webrtc/modules/audio_processing/BUILD.gn

Index: webrtc/modules/audio_processing/BUILD.gn
diff --git a/webrtc/modules/audio_processing/BUILD.gn b/webrtc/modules/audio_processing/BUILD.gn
index 5686dc66908dced9a188cb2929b8cf2ee035667d..bdcaeb4ba9d24c45bdc48d93919bd90f332240a9 100644
--- a/webrtc/modules/audio_processing/BUILD.gn
+++ b/webrtc/modules/audio_processing/BUILD.gn
@@ -500,6 +500,7 @@ if (rtc_include_tests) {
       "config_unittest.cc",
       "echo_cancellation_impl_unittest.cc",
       "splitting_filter_unittest.cc",
+      "test/analog_volume_mapper_unittest.cc",
       "transient/dyadic_decimator_unittest.cc",
       "transient/file_utils.cc",
       "transient/file_utils.h",
@@ -522,6 +523,7 @@ if (rtc_include_tests) {
     ]
 
     deps = [
+      ":analog_volume_mapper",
       ":audio_processing",
       ":audioproc_test_utils",
       "../..:webrtc_common",
@@ -670,6 +672,16 @@ if (rtc_include_tests) {
     }
   }
 
+  rtc_source_set("analog_volume_mapper") {
+    sources = [
+      "test/analog_volume_mapper.h",
+    ]
+
+    deps = [
+      "../../base:rtc_base_approved",
+    ]
+  }
+
   if (rtc_enable_protobuf) {
     rtc_executable("unpack_aecdump") {
       testonly = true