Let default device in PulseAudio be the system default device

media/audio/pulse/pulse_input.h

Index: media/audio/pulse/pulse_input.h
diff --git a/media/audio/pulse/pulse_input.h b/media/audio/pulse/pulse_input.h
index e0b2be9e2ef31134f7680b59e82956708844ca7f..b2de54fde76147fd4e491a13fd46c9638858dcb8 100644
--- a/media/audio/pulse/pulse_input.h
+++ b/media/audio/pulse/pulse_input.h
@@ -41,6 +41,9 @@ class PulseAudioInputStream : public AgcAudioStream<AudioInputStream> {
   double GetVolume() override;
   bool IsMuted() override;
 
+  pa_threaded_mainloop* GetPAMainloop();
+  void SetDefaultSystemDeviceName(const std::string& name);
+
  private:
   // PulseAudio Callbacks.
   static void ReadCallback(pa_stream* handle, size_t length, void* user_data);
@@ -52,6 +55,9 @@ class PulseAudioInputStream : public AgcAudioStream<AudioInputStream> {
                            int error,
                            void* user_data);
 
+  // Get default system input device for the input stream.
+  void GetSystemDefaultInputDevice();
+
   // Helper for the ReadCallback.
   void ReadData();
 
@@ -60,7 +66,12 @@ class PulseAudioInputStream : public AgcAudioStream<AudioInputStream> {
 
   AudioManagerPulse* audio_manager_;
   AudioInputCallback* callback_;
-  std::string device_name_;
+  const std::string device_name_;
+  // The name of the system default device. Set by
+  // GetSystemDefaultInputDeviceCallback if |device_name_| is set to be the
+  // default device.
+  std::string default_system_device_name_;
+
   AudioParameters params_;
   int channels_;
   double volume_;