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/src/java/org/webrtc/RefCountDelegate.java

@@ -0,0 +1,41 @@
+/*
+ *  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;
+
+import java.util.concurrent.atomic.AtomicInteger;
+import javax.annotation.Nullable;
+
+/**
+ * Implementation of RefCounted that executes a Runnable once the ref count reaches zero.
+ */
+class RefCountDelegate implements RefCounted {
+  private final AtomicInteger refCount = new AtomicInteger(1);
+  private final @Nullable Runnable releaseCallback;
+
+  /**
+   * @param releaseCallback Callback that will be executed once the ref count reaches zero.
+   */
+  public RefCountDelegate(@Nullable Runnable releaseCallback) {
+    this.releaseCallback = releaseCallback;
+  }
+
+  @Override
+  public void retain() {
+    refCount.incrementAndGet();
+  }
+
+  @Override
+  public void release() {
+    if (refCount.decrementAndGet() == 0 && releaseCallback != null) {
+      releaseCallback.run();
+    }
+  }
+}