Introduce PPB_AudioInput_Dev v0.3 and refactor the device enumeration code:

ppapi/cpp/dev/audio_input_dev.h

Index: ppapi/cpp/dev/audio_input_dev.h
diff --git a/ppapi/cpp/dev/audio_input_dev.h b/ppapi/cpp/dev/audio_input_dev.h
index fc336a946f750e5c8cce65f6e73d59adea631052..22408cdffb2230702badaa84d6f887e20d0db85d 100644
--- a/ppapi/cpp/dev/audio_input_dev.h
+++ b/ppapi/cpp/dev/audio_input_dev.h
@@ -37,6 +37,9 @@ class AudioInput_Dev : public Resource {
       const CompletionCallbackWithOutput<std::vector<DeviceRef_Dev> >&
           callback);
 
+  int32_t MonitorDeviceChange(PP_MonitorDeviceChangeCallback callback,
+                              void* user_data);
+
   /// If |device_ref| is null (i.e., is_null() returns true), the default device
   /// will be used.
   int32_t Open(const DeviceRef_Dev& device_ref,