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..6b7135361042df095f8026aa72044a8e17915c14 100644
--- a/webrtc/sdk/android/api/org/webrtc/AudioTrack.java
+++ b/webrtc/sdk/android/api/org/webrtc/AudioTrack.java
@@ -15,4 +15,13 @@ public class AudioTrack extends MediaStreamTrack {
   public AudioTrack(long nativeTrack) {
     super(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(super.nativeTrack, volume);
+  }
+
+  private static native void nativeSetVolume(long nativeTrack, double volume);
 }