Add support for adding VideoSinks to VideoTracks.

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

Index: webrtc/sdk/android/api/org/webrtc/VideoSink.java
diff --git a/webrtc/sdk/android/api/org/webrtc/VideoSink.java b/webrtc/sdk/android/api/org/webrtc/VideoSink.java
new file mode 100644
index 0000000000000000000000000000000000000000..e4162feb8b66f87181500bdbd5282a6151fb266e
--- /dev/null
+++ b/webrtc/sdk/android/api/org/webrtc/VideoSink.java
@@ -0,0 +1,23 @@
+/*
+ *  Copyright 2017 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 version of rtc::VideoSinkInterface.
+ */
+public interface VideoSink {
+  /**
+   * Implementations should call frame.retain() if they need to hold a reference to the frame after
+   * this function returns. Each call to retain() should be followed by a call to frame.release()
+   * when the reference is no longer needed.
+   */
+  void onFrame(VideoFrame frame);
+}