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.cc b/webrtc/api/mediaconstraintsinterface.cc
index 7ac7417..14a92c2 100644
--- a/webrtc/api/mediaconstraintsinterface.cc
+++ b/webrtc/api/mediaconstraintsinterface.cc
@@ -49,6 +49,8 @@
 const char MediaConstraintsInterface::kIntelligibilityEnhancer[] =
     "intelligibilityEnhancer";
 const char MediaConstraintsInterface::kLevelControl[] = "levelControl";
+const char MediaConstraintsInterface::kLevelControlInitialPeakLevelDBFS[] =
+    "levelControlInitialPeakLevelDBFS";
 const char MediaConstraintsInterface::kHighpassFilter[] =
     "googHighpassFilter";
 const char MediaConstraintsInterface::kTypingNoiseDetection[] =