This CL adds functionality in the level controller to

receive a signal level to use initially, instead of the
default initial signal level.

The initial form of the CL
(https://codereview.webrtc.org/2254973003/) was reverted
due to down-stream  dependencies. These have been resolved,
but the CL needed to be revised according to the new scheme
for passing parameters to the audio processing module.
Therefore, please review this CL as if it is new.

TBR=aleloi@webrtc.org
BUG=webrtc:6386

Review-Url: https://codereview.webrtc.org/2337083002
Cr-Commit-Position: refs/heads/master@{#14579}

webrtc/modules/audio_processing/include/audio_processing.h

@@ -252,6 +252,11 @@ class AudioProcessing {
   struct Config {
     struct LevelController {
       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].
+      float initial_peak_level_dbfs = -6.0206f;
     } level_controller;
   };