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..4d087d2d16ecbb9d824ee24860c54dd3dba0e2aa 100644
--- a/media/audio/pulse/pulse_input.h
+++ b/media/audio/pulse/pulse_input.h
@@ -52,6 +52,16 @@ class PulseAudioInputStream : public AgcAudioStream<AudioInputStream> {
                            int error,
                            void* user_data);
 
+  // pa_context_get_server_info callback. It's used by
+  // GetSystemDefaultInputDevice to set |default_system_device_name_| to the
+  // default system input device.
+  static void GetSystemDefaultInputDeviceCallback(pa_context* context,
+                                                  const pa_server_info* info,
+                                                  void* user_data);
+
+  // Get default system input device for the input stream.
+  void GetSystemDefaultInputDevice();
+
   // Helper for the ReadCallback.
   void ReadData();
 
@@ -60,7 +70,12 @@ 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_;