Add thread safety to AudioManagerBase to protect access to the audio thread member variable.

media/audio/audio_output_proxy.h

Index: media/audio/audio_output_proxy.h
diff --git a/media/audio/audio_output_proxy.h b/media/audio/audio_output_proxy.h
index af081c1d694b8cd9007a8e6e319d5982d8a3fee4..41e74e995005c30a04956e4104ed7e747bcf78b6 100644
--- a/media/audio/audio_output_proxy.h
+++ b/media/audio/audio_output_proxy.h
@@ -8,6 +8,7 @@
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
+#include "base/threading/non_thread_safe.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_parameters.h"
 
@@ -21,7 +22,9 @@ class AudioOutputDispatcher;
 //
 // AudioOutputProxy uses AudioOutputDispatcher to open and close
 // physical output streams.
-class MEDIA_EXPORT AudioOutputProxy : public AudioOutputStream {
+class MEDIA_EXPORT AudioOutputProxy
+  : public AudioOutputStream,
+    public NON_EXPORTED_BASE(base::NonThreadSafe) {
  public:
   // Caller keeps ownership of |dispatcher|.
   explicit AudioOutputProxy(AudioOutputDispatcher* dispatcher);