Eliminate usage of InterfacePtr::WaitForIncomingResponse.

components/filesystem/public/interfaces/file_system.mojom

 // Copyright 2015 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.
 
 module filesystem.mojom;
 
 import "components/filesystem/public/interfaces/directory.mojom";
 import "components/filesystem/public/interfaces/types.mojom";
 
 interface FileSystem {
   // Opens a temporary filesystem. Will return a different directory each time
   // it is called.
+  [Sync]
   OpenTempDirectory(Directory& directory) => (FileError error);
 
   // Returns a directory which will persist to disk.
+  [Sync]
   OpenPersistentFileSystem(Directory& directory) => (FileError error);
 };