Added support for changing the volume of AudioTrack as discussed in BUG=webrtc:6533

webrtc/sdk/android/api/org/webrtc/AudioTrack.java

 /*
  *  Copyright 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 package org.webrtc;
 
 /** Java wrapper for a C++ AudioTrackInterface */
 public class AudioTrack extends MediaStreamTrack {
+  final long nativeTrack;

[magjed_webrtc, 2017/04/05 19:14:00]

No need to introduce a new member variable, just use the nativeTrack in the
super class MediaStreamTrack instead.

+
   public AudioTrack(long nativeTrack) {
     super(nativeTrack);
+    this.nativeTrack = nativeTrack;
   }
+
+  /** Sets the volume for the underlying MediaSource. Volume is a gain value in the range
+   *  0 to 10.
+   */
+  public void setVolume(double volume) {
+    nativeSetVolume(nativeTrack, volume);
+  }
+
+  private static native void nativeSetVolume(long nativeTrack, double volume);
 }