Added a level controller initialization value to MediaConstraints.

An audio track with a level controller with the correct initialization
value can be created by a combination of
PeerConnectionFactory::CreateAudioTrack(..., audio_source) and
either
audio_source = PeerConnectionFactory::CreateAudioSource(constraints) or
audio_source = PeerConnectionFactory::CreateAudioSource(audio_options).

NOTRY=True
BUG=webrtc:6386

Review-Url: https://codereview.webrtc.org/2408143003
Cr-Commit-Position: refs/heads/master@{#14693}
diff --git a/webrtc/api/mediaconstraintsinterface.h b/webrtc/api/mediaconstraintsinterface.h
index d6fd6be..fbb1955 100644
--- a/webrtc/api/mediaconstraintsinterface.h
+++ b/webrtc/api/mediaconstraintsinterface.h
@@ -75,6 +75,8 @@
   static const char kExperimentalNoiseSuppression[];  // googNoiseSuppression2
   static const char kIntelligibilityEnhancer[];  // intelligibilityEnhancer
   static const char kLevelControl[];             // levelControl
+  static const char
+      kLevelControlInitialPeakLevelDBFS[];  // levelControlInitialPeakLevelDBFS
   static const char kHighpassFilter[];  // googHighpassFilter
   static const char kTypingNoiseDetection[];  // googTypingNoiseDetection
   static const char kAudioMirroring[];  // googAudioMirroring