This CL adds activation logic of the new APM level control
functionality and exposes the functionality using the
MediaConstraints.

The exposing of the feature through the  MediaConstraints
was done similarly to what was done for the intelligibility
enhancer in the CL
https://codereview.webrtc.org/1952123003

This CL is dependent on the CL https://codereview.webrtc.org/2090583002/ which contains
the level control functionality.

NOTRY=true
BUG=webrtc:5920

Review-Url: https://codereview.webrtc.org/2095563002
Cr-Commit-Position: refs/heads/master@{#13336}
diff --git a/webrtc/api/mediaconstraintsinterface.h b/webrtc/api/mediaconstraintsinterface.h
index 13560dd..d6fd6be 100644
--- a/webrtc/api/mediaconstraintsinterface.h
+++ b/webrtc/api/mediaconstraintsinterface.h
@@ -74,6 +74,7 @@
   static const char kNoiseSuppression[];  // googNoiseSuppression
   static const char kExperimentalNoiseSuppression[];  // googNoiseSuppression2
   static const char kIntelligibilityEnhancer[];  // intelligibilityEnhancer
+  static const char kLevelControl[];             // levelControl
   static const char kHighpassFilter[];  // googHighpassFilter
   static const char kTypingNoiseDetection[];  // googTypingNoiseDetection
   static const char kAudioMirroring[];  // googAudioMirroring