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

media/audio/linux/alsa_output_unittest.cc

Index: media/audio/linux/alsa_output_unittest.cc
diff --git a/media/audio/linux/alsa_output_unittest.cc b/media/audio/linux/alsa_output_unittest.cc
index 5b8405a3a335b20a91b8ac197d991e9cd0bda541..6a99ce07a7ba290110ab3f07a34f064e0c73801a 100644
--- a/media/audio/linux/alsa_output_unittest.cc
+++ b/media/audio/linux/alsa_output_unittest.cc
@@ -88,8 +88,8 @@ class MockAudioManagerLinux : public AudioManagerLinux {
 
   // We don't mock this method since all tests will do the same thing
   // and use the current message loop.
-  virtual MessageLoop* GetMessageLoop() {
-    return MessageLoop::current();
+  virtual scoped_refptr<base::MessageLoopProxy> GetMessageLoop() OVERRIDE {
+    return MessageLoop::current()->message_loop_proxy();
   }
 };