Android: Fix synchronization problems in VideoFrame release.

webrtc/sdk/android/src/java/org/webrtc/I420BufferImpl.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;
 
 import java.nio.ByteBuffer;
 import org.webrtc.VideoFrame.I420Buffer;
 
 /** Implementation of an I420 VideoFrame buffer. */
 class I420BufferImpl implements VideoFrame.I420Buffer {
   private final int width;
   private final int height;
   private final ByteBuffer dataY;
   private final ByteBuffer dataU;
   private final ByteBuffer dataV;
   private final int strideY;
   private final int strideU;
   private final int strideV;
   private final Runnable releaseCallback;
+  private final Object refCountLock = new Object();
 
   private int refCount;
 
   /** Constructs an I420Buffer backed by existing data. */
   I420BufferImpl(int width, int height, ByteBuffer dataY, int strideY, ByteBuffer dataU,
       int strideU, ByteBuffer dataV, int strideV, Runnable releaseCallback) {
     this.width = width;
     this.height = height;
     this.dataY = dataY;
     this.dataU = dataU;
@@ skipping 72 matching lines @@
   }
 
   @Override
   public I420Buffer toI420() {
     retain();
     return this;
   }
 
   @Override
   public void retain() {
-    ++refCount;
+    synchronized (refCountLock) {
+      ++refCount;
+    }
   }
 
   @Override
   public void release() {
-    if (--refCount == 0 && releaseCallback != null) {
-      releaseCallback.run();
+    synchronized (refCountLock) {
+      if (--refCount == 0 && releaseCallback != null) {
+        releaseCallback.run();
+      }
     }
   }
 
   @Override
   public VideoFrame.Buffer cropAndScale(
       int cropX, int cropY, int cropWidth, int cropHeight, int scaleWidth, int scaleHeight) {
     return VideoFrame.cropAndScaleI420(
         this, cropX, cropY, cropWidth, cropHeight, scaleWidth, scaleHeight);
   }
 }