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

ui/file_manager/file_manager/background/js/volume_manager_factory.js

Index: ui/file_manager/file_manager/background/js/volume_manager_factory.js
diff --git a/ui/file_manager/file_manager/background/js/volume_manager_factory.js b/ui/file_manager/file_manager/background/js/volume_manager_factory.js
new file mode 100644
index 0000000000000000000000000000000000000000..013de01b94c9dc72a17c523e6d08a524eadf15ed
--- /dev/null
+++ b/ui/file_manager/file_manager/background/js/volume_manager_factory.js
@@ -0,0 +1,64 @@
+// 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 = (function() {
+  /**
+   * The singleton instance of VolumeManager. Initialized by the first
+   * invocation of getInstance().
+   * @type {VolumeManager}
+   */
+  var instance = null;
+
+  /**
+   * @type {Promise}
+   */
+  var instancePromise = null;
+
+  /**
+   * Returns the VolumeManager instance asynchronously. If it has not been
+   * created or is under initialization, it will waits for the finish of the
+   * initialization.
+   * @param {function(VolumeManager)=} opt_callback Called with the
+   *     VolumeManager instance. TODO(hirono): Remove the callback and use
+   *     Promise instead.
+   * @return {Promise} Promise to be fulfilled with the volume manager.
+   */
+  function getInstance(opt_callback) {
+    if (!instancePromise) {
+      instance = new VolumeManagerImpl();
+      instancePromise = new Promise(function(fulfill) {
+        instance.initialize_(function() {
+          return fulfill(instance);
+        });
+      });
+    }
+    if (opt_callback)
+      instancePromise.then(opt_callback);
+    return instancePromise;
+  };
+
+  /**
+   * Returns instance of VolumeManager for debug purpose.
+   * This method returns VolumeManager.instance which may not be initialized.
+   *
+   * @return {VolumeManager} Volume manager.
+   */
+  function getInstanceForDebug() {
+    return instance;
+  };
+
+  /**
+   * Revokes the singleton instance for testing.
+   */
+  function revokeInstanceForTesting() {
+    instancePromise = null;
+    instance = null;
+  };
+
+  return {
+    getInstance: getInstance,
+    getInstanceForDebug: getInstanceForDebug,
+    revokeInstanceForTesting: revokeInstanceForTesting
+  };
+}());