Reland of added functionality for specifying the initial signal level to use for the gain estimation

webrtc/modules/audio_processing/include/audio_processing.h

Index: webrtc/modules/audio_processing/include/audio_processing.h
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index 035fa32c7ad22121eb56d832ffc599a5602659d9..7a3439d98667abd412f606d72ce8404c8a5c1f3a 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -20,6 +20,7 @@
 #include <vector>
 
 #include "webrtc/base/arraysize.h"
+#include "webrtc/base/optional.h"
 #include "webrtc/base/platform_file.h"
 #include "webrtc/modules/audio_processing/beamformer/array_util.h"
 #include "webrtc/modules/audio_processing/include/config.h"
@@ -251,7 +252,14 @@ class AudioProcessing {
   // The config is applied by passing the struct to the ApplyConfig method.
   struct Config {
     struct LevelController {
+      LevelController();
+      ~LevelController();

[the sun, 2016/09/14 10:00:59]

is the dtor really necessary?

[peah-webrtc, 2016/09/16 07:11:07]

clang complains if I remove it. So I guess it is needed.

       bool enabled = false;
+
+      // Sets the initial peak level to use inside the level controller in order
+      // to compute the signal gain. The unit for the peak level is dBFS and
+      // the allowed range is [-100, 0].
+      rtc::Optional<float> initial_level;

[the sun, 2016/09/14 10:00:59]

Why don't we always have an initial level?

[peah-webrtc, 2016/09/16 07:11:06]

We do. One way would be to hardcode the default initial value as a default
parameter, but I thought it was better to hardcode the default initial value as
a constant that is local to the level controller. I thought it made sense to
gather the code that controls the LevelController behavior in one place.

WDYT?

[the sun, 2016/09/16 08:00:40]

If I picture myself as someone who'd consider using the LC, I'd actually prefer
having the default value right here in the Config. For one thing, if the default
is changed down the line, it'd be easier for me to spot that, since it would be
visible as an interface change.

[peah-webrtc, 2016/09/16 11:36:05]

That is true. I agree.

     } level_controller;
   };