Factor out AudioOutputDelegate from AudioRendererHost.

content/browser/audio_device_thread.cc

+// Copyright 2016 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 "base/threading/thread_task_runner_handle.h"
+#include "build/build_config.h"
+#include "content/browser/audio_device_thread.h"
+
+namespace content {
+
+scoped_refptr<base::SingleThreadTaskRunner> AudioDeviceThread::GetTaskRunner() {
+#if defined(OS_MACOSX)
+    // On Mac audio task runner must belong to the main thread.
+    // See http://crbug.com/158170.
+    return base::ThreadTaskRunnerHandle::Get();
+#else
+    return thread_.task_runner();
+#endif  // defined(OS_MACOSX)
+  }
+
+AudioDeviceThread::AudioDeviceThread() : thread_("AudioThread") {
+#if defined(OS_WIN)
+  thread_.init_com_with_mta(true);
+#endif
+  CHECK(thread_.Start());
+}
+
+}  // namespace content