Add a new audio extension event OnLevelChanged.

extensions/test/data/api_test/audio/volume_change/background.js

Index: extensions/test/data/api_test/audio/volume_change/background.js
diff --git a/extensions/test/data/api_test/usb/add_event/background.js b/extensions/test/data/api_test/audio/volume_change/background.js
similarity index 56%
copy from extensions/test/data/api_test/usb/add_event/background.js
copy to extensions/test/data/api_test/audio/volume_change/background.js
index 1f8382a8e81a7bd4232a93667fe3747efd85f00e..44a17c49042b680437dfdc35c5dfabda3c2941bf 100644
--- a/extensions/test/data/api_test/usb/add_event/background.js
+++ b/extensions/test/data/api_test/audio/volume_change/background.js
@@ -2,12 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-chrome.usb.onDeviceAdded.addListener(function(device) {
-  if (device.vendorId == 6353 && device.productId == 22768) {
+chrome.audio.OnOutputNodeVolumeChanged.addListener(function (id, volume) {
+  if (id == 30001 && volume == 60) {
     chrome.test.sendMessage("success");
   } else {
-    console.error("Got unexpected device: vid:" + device.vendorId +
-                  " pid:" + device.productId);
+    console.error("Got unexpected OnOutputNodeVolumeChanged event id:" + id +
+                  " volume:" + volume);
     chrome.test.sendMessage("failure");
   }
 });