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

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

Index: webrtc/sdk/android/api/org/webrtc/AudioTrack.java
diff --git a/webrtc/sdk/android/api/org/webrtc/AudioTrack.java b/webrtc/sdk/android/api/org/webrtc/AudioTrack.java
index c33a9d8db1d83aa1988520119361afc7284c414d..f7fc6881079dae33f34722c0d1cd8def79ee07f4 100644
--- a/webrtc/sdk/android/api/org/webrtc/AudioTrack.java
+++ b/webrtc/sdk/android/api/org/webrtc/AudioTrack.java
@@ -12,7 +12,16 @@ package org.webrtc;
 
 /** Java wrapper for a C++ AudioTrackInterface */
 public class AudioTrack extends MediaStreamTrack {
+  final long nativeTrack;
+
   public AudioTrack(long nativeTrack) {
     super(nativeTrack);
+    this.nativeTrack = nativeTrack;
+  }
+
+  public AudioSource getSource() {
+    return nativeGetSource(nativeTrack);
   }
+
+  private static native AudioSource nativeGetSource(long nativeTrack);
 }