Android: Add common function for adding/releasing native reference

webrtc/sdk/android/src/java/org/webrtc/JniCommon.java

+/*
+ *  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;
+
+/** Class with static JNI helper functions that are used in many places. */
+class JniCommon {
+  static native void nativeAddRef(long nativeRefCountedPointer);

[sakal, 2017/08/25 13:19:39]

nit: comment that the pointer is a reference to rtc::RefCountInterface*

[magjed_webrtc, 2017/08/27 20:44:32]

Done.

+  static native void nativeReleaseRef(long nativeRefCountedPointer);
+}