Add the chrome.fileSystem.onVolumeListChanged event.

chrome/test/data/extensions/api_test/file_system/on_volume_list_changed/background.js

Index: chrome/test/data/extensions/api_test/file_system/on_volume_list_changed/background.js
diff --git a/chrome/test/data/extensions/api_test/file_system/on_volume_list_changed/background.js b/chrome/test/data/extensions/api_test/file_system/on_volume_list_changed/background.js
new file mode 100644
index 0000000000000000000000000000000000000000..2dba1c3ecd4012bc83ff296548bc51f3c3ff6f7c
--- /dev/null
+++ b/chrome/test/data/extensions/api_test/file_system/on_volume_list_changed/background.js
@@ -0,0 +1,27 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+function runTests() {
+  chrome.test.runTests([
+    function onVolumeListChanged() {
+      chrome.fileSystem.getVolumeList(
+         chrome.test.callbackPass(function(volumeList) {
+           chrome.test.assertEq(4, volumeList.length);
+
+           // Confirm that adding a newly mounted volume emits an event, and
+           // that the volume list is updated.
+           chrome.fileSystem.onVolumeListChanged.addListener(
+               chrome.test.callbackPass(function(event) {
+                 chrome.test.assertEq(5, event.volumes.length);
+                 chrome.fileSystem.getVolumeList(
+                     chrome.test.callbackPass(function(volumeList) {
+                       chrome.test.assertEq(5, volumeList.length);
+                     }));
+               }));
+      }));
+    }
+  ]);
+}
+
+chrome.app.runtime.onLaunched.addListener(runTests);