use lock to make sure device will not deliver any frame after it's stopped.

media/video/capture/mac/video_capture_device_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.
 
 // OS X implementation of VideoCaptureDevice, using QTKit as native capture API.
 
 #ifndef MEDIA_VIDEO_CAPTURE_MAC_VIDEO_CAPTURE_DEVICE_MAC_H_
 #define MEDIA_VIDEO_CAPTURE_MAC_VIDEO_CAPTURE_DEVICE_MAC_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
+#include "base/synchronization/lock.h"
 #include "media/video/capture/video_capture_device.h"
 #include "media/video/capture/video_capture_types.h"
 
 @class VideoCaptureDeviceQTKit;
 
 namespace media {
 
 // Called by VideoCaptureManager to open, close and start, stop video capture
 // devices.
 class VideoCaptureDeviceMac : public VideoCaptureDevice {
@@ skipping 25 matching lines @@
     kNotInitialized,
     kIdle,
     kAllocated,
     kCapturing,
     kError
   };
 
   VideoCaptureDevice::Name device_name_;
   VideoCaptureDevice::EventHandler* observer_;
   InternalState state_;
+  base::Lock lock_;
 
   VideoCaptureDeviceQTKit* capture_device_;
 
   DISALLOW_COPY_AND_ASSIGN(VideoCaptureDeviceMac);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_VIDEO_CAPTURE_MAC_VIDEO_CAPTURE_DEVICE_MAC_H_