Add helper classes to for managing shared buffers.

mojo/services/media/common/cpp/shared_media_buffer_allocator.cc

+// Copyright 2015 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 "mojo/services/media/common/cpp/shared_media_buffer_allocator.h"
+
+namespace mojo {
+namespace media {
+
+SharedMediaBufferAllocator::~SharedMediaBufferAllocator() {
+  std::lock_guard<std::mutex> lock(lock_);
+}
+
+void SharedMediaBufferAllocator::OnInit() {
+  std::lock_guard<std::mutex> lock(lock_);
+  fifo_allocator_.Reset(size());
+}
+
+} // namespace media
+} // namespace mojo