Fix suspicious code detected by closure compiler.

chrome/common/extensions/api/file_manager_private.idl

 // Copyright 2013 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.
 
 // fileManagerPrivate API.
 // This is a private API used by the file browser of ChromeOS.
 [platforms=("chromeos"),
  implemented_in="chrome/browser/chromeos/extensions/file_manager/file_manager_private_api_functions.h"]
 namespace fileManagerPrivate {
 // Type of the mounted volume.
@@ skipping 647 matching lines @@
   // Unmounts a mounted resource.
   // |volumeId| An ID of the volume.
   static void removeMount(DOMString volumeId);
 
   // Get the list of mounted volumes.
   // |callback|
   static void getVolumeMetadataList(GetVolumeMetadataListCallback callback);
 
   // Cancels ongoing file transfers for selected files.
   // |fileUrls| Array of files for which ongoing transfer should be canceled.
-  //     If this is absent, all jobs are canceled. 
-  // |callback|
-  static void cancelFileTransfers(optional DOMString[] fileUrls,
-                                  CancelFileTransfersCallback callback);
+  //     If this is absent, all jobs are canceled.
+  // |callback| Completion callback of the cancel.
+  static void cancelFileTransfers(
+      optional DOMString[] fileUrls,
+      optional CancelFileTransfersCallback callback);
 
   // Starts to copy an entry. If the source is a directory, the copy is done
   // recursively.
   // |sourceUrl| URL of the source entry to be copied.
   // |parent| URL of the destination directory.
   // |newName| Name of the new entry. It shouldn't contain '/'.
   // |callback| Completion callback.
   static void startCopy(DOMString sourceUrl,
                         DOMString parent,
                         DOMString newName,
@@ skipping 121 matching lines @@
 
   static void onPreferencesChanged();
 
   static void onDriveConnectionStatusChanged();
 
   static void onDeviceChanged(DeviceEvent event);
 
   static void onDriveSyncError(DriveSyncErrorEvent event);
 };
 };