Add PeerConnectionObserver#onRemoveTrack to android sdk

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

Index: webrtc/sdk/android/api/org/webrtc/PeerConnection.java
diff --git a/webrtc/sdk/android/api/org/webrtc/PeerConnection.java b/webrtc/sdk/android/api/org/webrtc/PeerConnection.java
index b2155c42e4d0e79561adb746a3ac67a989fbc179..ecc1c8656f2fea0af2ec0ffc688c4556a502a618 100644
--- a/webrtc/sdk/android/api/org/webrtc/PeerConnection.java
+++ b/webrtc/sdk/android/api/org/webrtc/PeerConnection.java
@@ -91,7 +91,13 @@ public class PeerConnection {
      * Triggered when a new track is signaled by the remote peer, as a result of
      * setRemoteDescription.
      */
-    public void onAddTrack(RtpReceiver receiver, MediaStream[] mediaStreams);

[Taylor Brandstetter, 2017/08/29 19:48:09]

It's not safe to change the signature of this; existing applications may be
depending on it.

[korniltsev, 2017/08/30 21:47:47]

reverted it back

+    public void onAddTrack(MediaStreamTrack track, MediaStream[] mediaStreams);
+
+    /**
+     * Triggered when a previously added remote track is removed by the remote peer,
+     * as a result of setRemoteDescription.
+     */
+    void onRemoveTrack(MediaStreamTrack track, MediaStream[] mediaStreams);

[Taylor Brandstetter, 2017/08/29 19:48:09]

Should be public. Also, can you give it an empty default implementation so that
this change doesn't break existing Observer subclasses? I believe this is
possible now, since the chromium build infrastructure recently started
supporting Java 8 features.

[korniltsev, 2017/08/30 21:47:47]

Done.

   }
 
   /** Java version of PeerConnectionInterface.IceServer. */