Divide volume_manager.js into files for each classes and extract interfaces from them.

ui/file_manager/externs/background_window_common.js

Index: ui/file_manager/externs/background_window_common.js
diff --git a/ui/file_manager/externs/background_window_common.js b/ui/file_manager/externs/background_window_common.js
new file mode 100644
index 0000000000000000000000000000000000000000..201f00d757a21c55aada387899082559020edf1f
--- /dev/null
+++ b/ui/file_manager/externs/background_window_common.js
@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+var volumeManagerFactory = {};
+
+/**
+ * @param {function(VolumeManager)=} opt_callback
+ * @return {Promise}
+ */
+volumeManagerFactory.getInstance = function(opt_callback) {};
+
+/**
+ * @return {VolumeManager}
+ */
+volumeManagerFactory.getInstanceForDebug = function() {};
+
+volumeManagerFactory.revokeInstanceForTesting = function() {};