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

media/video/capture/mac/video_capture_device_mac.h

Index: media/video/capture/mac/video_capture_device_mac.h
===================================================================
--- media/video/capture/mac/video_capture_device_mac.h	(revision 155213)
+++ media/video/capture/mac/video_capture_device_mac.h	(working copy)
@@ -10,6 +10,7 @@
 #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"
 
@@ -55,6 +56,7 @@
   VideoCaptureDevice::Name device_name_;
   VideoCaptureDevice::EventHandler* observer_;
   InternalState state_;
+  base::Lock lock_;
 
   VideoCaptureDeviceQTKit* capture_device_;