PPAPI: Force async callback invocation option.

ppapi/cpp/file_system.cc

 // Copyright (c) 2011 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.
 
 #include "ppapi/cpp/file_system.h"
 
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/c/ppb_file_system.h"
 #include "ppapi/cpp/completion_callback.h"
 #include "ppapi/cpp/file_ref.h"
@@ skipping 14 matching lines @@
                        PP_FileSystemType type) {
   if (!has_interface<PPB_FileSystem>())
     return;
   PassRefFromConstructor(get_interface<PPB_FileSystem>()->Create(
       instance->pp_instance(), type));
 }
 
 int32_t FileSystem::Open(int64_t expected_size,
                          const CompletionCallback& cc) {
   if (!has_interface<PPB_FileSystem>())
-    return PP_ERROR_NOINTERFACE;
+    return cc.MayForce(PP_ERROR_NOINTERFACE);
   return get_interface<PPB_FileSystem>()->Open(
       pp_resource(), expected_size, cc.pp_completion_callback());
 }
 
 }  // namespace pp