Listen for OutputNodeVolumeChanged signal

chromeos/dbus/cras_audio_client.h

Index: chromeos/dbus/cras_audio_client.h
diff --git a/chromeos/dbus/cras_audio_client.h b/chromeos/dbus/cras_audio_client.h
index 3851e516a19957dc2b82b54b3e4f7732dac8e5c1..54284be264527ddf2c489aff2ccea02a2e2678b2 100644
--- a/chromeos/dbus/cras_audio_client.h
+++ b/chromeos/dbus/cras_audio_client.h
@@ -42,6 +42,9 @@ class CHROMEOS_EXPORT CrasAudioClient : public DBusClient {
     // Called when active audio input node changed to new node with |node_id|.
     virtual void ActiveInputNodeChanged(uint64_t node_id);
 
+    // Called when output node's volume changed.
+    virtual void OutputNodeVolumeChanged(uint64_t node_id, int volume);
+
    protected:
     virtual ~Observer();
   };