Make VideoFrame.Buffer implementations lock-free.

Replaces lock-based implementation with AtomicInteger.

Bug: webrtc:7749
Change-Id: I226093b0af2090c080dfd4f87ed8f33a3f9efbd8
Reviewed-on: https://webrtc-review.googlesource.com/64162
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22798}

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

@@ -0,0 +1,28 @@
+/*
+ *  Copyright 2018 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;
+
+/**
+ * Interface for ref counted objects in WebRTC. These objects have significant resources that need
+ * to be freed when they are no longer in use. Each objects starts with ref count of one when
+ * created. If a reference is passed as a parameter to a method, the caller has ownesrship of the
+ * object by default - calling release is not necessary unless retain is called.
+ */
+public interface RefCounted {
+  /** Increases ref count by one. */
+  void retain();
+
+  /**
+   * Decreases ref count by one. When the ref count reaches zero, resources related to the object
+   * will be freed.
+   */
+  void release();
+}