Avoids crash in PCMWaveInAudioInputStream::WaveCallback

media/audio/win/wavein_input_win.h

Index: media/audio/win/wavein_input_win.h
===================================================================
--- media/audio/win/wavein_input_win.h	(revision 179519)
+++ media/audio/win/wavein_input_win.h	(working copy)
@@ -12,6 +12,7 @@
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/synchronization/lock.h"
 #include "base/win/scoped_handle.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_parameters.h"
@@ -116,6 +117,9 @@
   // An event that is signaled when the callback thread is ready to stop.
   base::win::ScopedHandle stopped_event_;
 
+  // Lock used to avoid conflicts when Stop() is called during a callback.
+  base::Lock lock_;
+
   DISALLOW_COPY_AND_ASSIGN(PCMWaveInAudioInputStream);
 };