Add a new audio extension event OnLevelChanged.

extensions/browser/api/audio/audio_service_linux.cc

Index: extensions/browser/api/audio/audio_service_linux.cc
diff --git a/extensions/browser/api/audio/audio_service_linux.cc b/extensions/browser/api/audio/audio_service_linux.cc
index 4e70cef40ffb096ebd823e495cf20e0f2d9de616..87825b70db9dce0b4e7ed77953d15288b1bc40b8 100644
--- a/extensions/browser/api/audio/audio_service_linux.cc
+++ b/extensions/browser/api/audio/audio_service_linux.cc
@@ -17,6 +17,7 @@ namespace extensions {
 
 using core_api::audio::OutputDeviceInfo;
 using core_api::audio::InputDeviceInfo;
+using core_api::audio::AudioDeviceInfo;
 
 class AudioServiceImpl : public AudioService {
  public:
@@ -35,6 +36,7 @@ class AudioServiceImpl : public AudioService {
                            bool muted,
                            int volume,
                            int gain) override;
+  void StartGetDeviceInfo(const GetDeviceInfoCallback& callback) override;
 
   // List of observers.
   ObserverList<AudioService::Observer> observer_list_;
@@ -84,6 +86,13 @@ bool AudioServiceImpl::SetDeviceProperties(const std::string& device_id,
   return false;
 }
 
+void AudioServiceImpl::StartGetDeviceInfo(
+    const GetDeviceInfoCallback& callback) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  if (!callback.is_null())
+    callback.Run(DevicesInfo(), false);
+}
+
 AudioService* AudioService::CreateInstance() {
   return new AudioServiceImpl;
 }