AudioManagerBase: Create and run the audio thread lazily.

media/audio/alsa/alsa_output_unittest.cc

Index: media/audio/alsa/alsa_output_unittest.cc
diff --git a/media/audio/alsa/alsa_output_unittest.cc b/media/audio/alsa/alsa_output_unittest.cc
index 908da6b61bb6ab84b47b485ad463cb514fbc77e3..1b1386347f9437c21865c9fa64b71936090ee735 100644
--- a/media/audio/alsa/alsa_output_unittest.cc
+++ b/media/audio/alsa/alsa_output_unittest.cc
@@ -95,7 +95,7 @@ class MockAudioManagerAlsa : public AudioManagerAlsa {
 
   // We don't mock this method since all tests will do the same thing
   // and use the current task runner.
-  scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() const override {
+  scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() override {
     return base::MessageLoop::current()->task_runner();
   }