Add the "virtual" keyword on method overrides that are missing it.

webkit/glue/plugins/pepper_file_system.cc

 // Copyright (c) 2010 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 "webkit/glue/plugins/pepper_file_system.h"
 
 #include "base/ref_counted.h"
 #include "ppapi/c/dev/ppb_file_system_dev.h"
 #include "ppapi/c/pp_completion_callback.h"
 #include "third_party/WebKit/WebKit/chromium/public/WebDocument.h"
@@ skipping 60 matching lines @@
 
 }  // namespace
 
 FileSystem::FileSystem(PluginInstance* instance, PP_FileSystemType_Dev type)
     : Resource(instance->module()),
       instance_(instance),
       type_(type),
       opened_(false) {
 }
 
+FileSystem* FileSystem::AsFileSystem() {
+  return this;
+}
+
 const PPB_FileSystem_Dev* FileSystem::GetInterface() {
   return &ppb_filesystem;
 }
 
 }  // namespace pepper