[fsp] Use types from another namespaces in IDL.

chrome/common/extensions/api/file_system_provider_internal.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.
 
 // Internal, used by fileSystemProvider's custom bindings. These functions are
 // called when events' callbacks are invoked.
 [platforms=("chromeos"),
  implemented_in="chrome/browser/chromeos/extensions/file_system_provider/file_system_provider_api.h"]
 namespace fileSystemProviderInternal {
   interface Functions {
     // Internal. Success callback of the <code>onUnmountRequested</code>
     // event. Must be called when unmounting is completed.
     static void unmountRequestedSuccess(long fileSystemId,
                                         long requestId);
 
     // Internal. Error callback of the <code>onUnmountRequested</code>
     // event. Must be called if unmounting fails.
     static void unmountRequestedError(
         long fileSystemId,
         long requestId,
-        // TODO(mtomasz): Use FileSystemProvider::ProviderError when namespaces
-        // are fully implemented in IDL.
-        DOMString error);
+        fileSystemProvider.ProviderError error);
   };
 };