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

media/audio/linux/audio_manager_linux.cc

Index: media/audio/linux/audio_manager_linux.cc
diff --git a/media/audio/linux/audio_manager_linux.cc b/media/audio/linux/audio_manager_linux.cc
index 6aaf59182d0d934f90098231c956fedb2c750fee..54f1346774d302690511e51a37f68e87e0f1e7fa 100644
--- a/media/audio/linux/audio_manager_linux.cc
+++ b/media/audio/linux/audio_manager_linux.cc
@@ -49,17 +49,9 @@ bool AudioManagerLinux::HasAudioInputDevices() {
 
 AudioOutputStream* AudioManagerLinux::MakeAudioOutputStream(
     const AudioParameters& params) {
-  // Early return for testing hook.  Do this before checking for
-  // |initialized_|.
-  if (params.format == AudioParameters::AUDIO_MOCK) {
+  // Early return for testing hook.
+  if (params.format == AudioParameters::AUDIO_MOCK)
     return FakeAudioOutputStream::MakeFakeStream(params);
-  }
-
-  if (!initialized()) {
-    // We should never get here since this method is called on the audio thread.
-    NOTREACHED();
-    return NULL;
-  }
 
   // Don't allow opening more than |kMaxOutputStreams| streams.
   if (active_output_stream_count_ >= kMaxOutputStreams)
@@ -97,9 +89,6 @@ AudioInputStream* AudioManagerLinux::MakeAudioInputStream(
     return FakeAudioInputStream::MakeFakeStream(params);
   }
 
-  if (!initialized())
-    return NULL;
-
   std::string device_name = (device_id == AudioManagerBase::kDefaultDeviceId) ?
       AlsaPcmInputStream::kAutoSelectDevice : device_id;
   if (CommandLine::ForCurrentProcess()->HasSwitch(switches::kAlsaInputDevice)) {