Add an unittest for background/volume_manager.js

ui/file_manager/file_manager/background/js/volume_manager.js

Index: ui/file_manager/file_manager/background/js/volume_manager.js
diff --git a/ui/file_manager/file_manager/background/js/volume_manager.js b/ui/file_manager/file_manager/background/js/volume_manager.js
index 9e3f2aa35858077f0d35b45f0ee2d6aa27dfb91d..695e87c16880fb8489f7d43e5350c93971e6216b 100644
--- a/ui/file_manager/file_manager/background/js/volume_manager.js
+++ b/ui/file_manager/file_manager/background/js/volume_manager.js
@@ -503,13 +503,6 @@ VolumeManager.prototype.__proto__ = cr.EventTarget.prototype;
 VolumeManager.TIMEOUT = 15 * 60 * 1000;
 
 /**
- * Queue to run getInstance sequentially.
- * @type {AsyncUtil.Queue}
- * @private
- */
-VolumeManager.getInstanceQueue_ = new AsyncUtil.Queue();
-
-/**
  * The singleton instance of VolumeManager. Initialized by the first invocation
  * of getInstance().
  * @type {VolumeManager}
@@ -555,6 +548,14 @@ VolumeManager.getInstance = function(opt_callback) {
 };
 
 /**
+ * Revokes the singleton instance for testing.
+ */
+VolumeManager.revokeInstanceForTesting = function() {
+  VolumeManager.instancePromise_ = null;
+  VolumeManager.instance_ = null;
+}
+
+/**
  * Initializes mount points.
  * @param {function()} callback Called upon the completion of the
  *     initialization.