NullAudioStream: Provide factory function

src/common/null_audio_stream.cpp

@@ -18,3 +18,8 @@ void NullAudioStream::BufferAvailable()
   // drop any buffer as soon as they're available
   DropBuffer();
 }
+
+std::unique_ptr<AudioStream> NullAudioStream::Create()
+{
+  return std::unique_ptr<AudioStream>(new NullAudioStream());
+}

src/common/null_audio_stream.h

@@ -4,7 +4,6 @@
 class NullAudioStream final : public AudioStream
 {
 public:
-  NullAudioStream();
   ~NullAudioStream();
 
   static std::unique_ptr<AudioStream> Create();
@@ -14,4 +13,7 @@ protected:
   void PauseDevice(bool paused) override;
   void CloseDevice() override;
   void BufferAvailable() override;
+
+private:
+  NullAudioStream();
 };