Mac AVFoundation: Enumerate devices in device/audio thread. Take 2.

content/browser/device_monitor_mac.h

 // Copyright (c) 2012 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.
 
 #ifndef CONTENT_BROWSER_DEVICE_MONITOR_MAC_H_
 #define CONTENT_BROWSER_DEVICE_MONITOR_MAC_H_
 
 #include "base/basictypes.h"
 #include "base/system_monitor/system_monitor.h"
 #include "base/threading/thread_checker.h"
 
 namespace {
 class DeviceMonitorMacImpl;
 }
 
 namespace content {
 
 // Class to track audio/video devices removal or addition via callback to
 // base::SystemMonitor ProcessDevicesChanged(). A single object of this class
 // is created from the browser main process and lives as long as this one.
 class DeviceMonitorMac {
  public:
   DeviceMonitorMac();
   ~DeviceMonitorMac();
 
   // Registers the observers for the audio/video device removal, connection and
   // suspension. The AVFoundation library is also loaded and initialised if the
-  // OS supports it.
-  void StartMonitoring();
+  // OS supports it. The |device_task_runner| argument represents the thread on
+  // which device enumeration will occur.
+  void StartMonitoring(
+    const scoped_refptr<base::SingleThreadTaskRunner>& device_task_runner);
 
   // Method called by the internal DeviceMonitorMacImpl object
   // |device_monitor_impl_| when a device of type |type| has been added to or
   // removed from the system. This code executes in the notification thread
   // (QTKit or AVFoundation).
   void NotifyDeviceChanged(base::SystemMonitor::DeviceType type);
 
  private:
   scoped_ptr<DeviceMonitorMacImpl> device_monitor_impl_;
 
   // |thread_checker_| is used to check that constructor and StartMonitoring()
-  // are called in the correct thread, that also owns the object.
+  // are called in the correct thread, the UI thread, that also owns the object.
   base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(DeviceMonitorMac);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_DEVICE_MONITOR_MAC_H_