Add a lock around getting and setting of source_ to avoid possible compiler reorderings.

media/audio/mac/audio_output_mac.h

Index: media/audio/mac/audio_output_mac.h
diff --git a/media/audio/mac/audio_output_mac.h b/media/audio/mac/audio_output_mac.h
index 031dc3f60d53b1e400711ad3c27e0da9183b3b1d..8d18c3eeb0dab720b508c96d4adcafe9795505c0 100644
--- a/media/audio/mac/audio_output_mac.h
+++ b/media/audio/mac/audio_output_mac.h
@@ -9,6 +9,7 @@
 #include <AudioToolbox/AudioQueue.h>
 #include <AudioUnit/AudioUnit.h>
 
+#include "base/synchronization/lock.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_parameters.h"
 
@@ -52,12 +53,19 @@ class PCMQueueOutAudioOutputStream : public AudioOutputStream {
   // Called when an error occurs.
   void HandleError(OSStatus err);
 
+  // Atomic operations for setting/getting the source callback.
+  void SetSource(AudioSourceCallback* source);
+  AudioSourceCallback* GetSource();
+
   // Structure that holds the stream format details such as bitrate.
   AudioStreamBasicDescription format_;
   // Handle to the OS audio queue object.
   AudioQueueRef audio_queue_;
   // Array of pointers to the OS managed audio buffers.
   AudioQueueBufferRef buffer_[kNumBuffers];
+  // Mutex for the |source_| to implment atomic set and get.
+  // It is important to NOT wait on any other locks while this is held.
+  base::Lock source_lock_;
   // Pointer to the object that will provide the audio samples.
   AudioSourceCallback* source_;
   // Our creator, the audio manager needs to be notified when we close.