MojoCompositorFrameSinkPrivate should support copy requests

cc/output/copy_output_result.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/copy_output_result.h"
 
 #include "base/logging.h"
 #include "cc/resources/texture_mailbox.h"
 
 namespace cc {
@@ skipping 26 matching lines @@
 
 void CopyOutputResult::TakeTexture(
     TextureMailbox* texture_mailbox,
     std::unique_ptr<SingleReleaseCallback>* release_callback) {
   *texture_mailbox = texture_mailbox_;
   *release_callback = std::move(release_callback_);
 
   texture_mailbox_ = TextureMailbox();
 }
 
+std::unique_ptr<SingleReleaseCallback> CopyOutputResult::TakeReleaseCallback() {
+  return std::move(release_callback_);
+}
+
 }  // namespace cc