[fsp] Simplify ID of provided file systems.

chrome/common/extensions/api/file_system_provider.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.
 
 // Use the <code>chrome.fileSystemProvider</code> API to create file systems,
 // that can be accessible from the file manager on Chrome OS.
 [platforms=("chromeos"),
  implemented_in="chrome/browser/chromeos/extensions/file_system_provider/file_system_provider_api.h"]
 namespace fileSystemProvider {
   // Callback to receive the result of mount() function.
   // <code>fileSystemID</code> will be a unique ID for the file system just
   // mounted. The ID is used to distinguish multiple file systems mounted
   // from a single File System Provider.
-  callback MountCallback = void(DOMString fileSystemId,
+  callback MountCallback = void(long fileSystemId,
                                 [nodoc, instanceOf=DOMError] object error);
 
   // Callback to handle an error raised from the browser.
   [nocompile] callback ErrorCallback = void([instanceOf=DOMError] object error);
 
   interface Functions {
     // Mounts a file system with the given <code>displayName</code>.
     // <code>displayName</code> will be shown in the left panel of
     // Files.app. <code>displayName</code> can contain any characters
     // including '/', but cannot be an empty string. <code>displayName</code>
     // should be descritive but doesn't have to be unique. Duplicate display
     // names are uniquified by adding suffix like "(1)" in the Files.app UI.
     static void mount(DOMString displayName,
                       MountCallback successCallback,
                       [nocompile] ErrorCallback errorCallback);
   };
 };