gdata: Make callback parameters mandatory for CloseFile functions

chrome/browser/chromeos/gdata/gdata_file_system_interface.h

 // Copyright (c) 2012 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_GDATA_GDATA_FILE_SYSTEM_INTERFACE_H_
 #define CHROME_BROWSER_CHROMEOS_GDATA_GDATA_FILE_SYSTEM_INTERFACE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 162 matching lines @@
   // Otherwise, GData file system does not pick up the file for uploading.
   //
   // Can be called from UI/IO thread. |callback| is run on the calling thread.
   virtual void OpenFile(const FilePath& file_path,
                         const OpenFileCallback& callback) = 0;
 
   // Closes a file at the virtual path |file_path| on the gdata file system,
   // which is opened via OpenFile(). It commits the dirty flag on the cache.
   //
   // Can be called from UI/IO thread. |callback| is run on the calling thread.
+  // |callback| must not be null.
   virtual void CloseFile(const FilePath& file_path,
                          const FileOperationCallback& callback) = 0;
 
   // Copies |src_file_path| to |dest_file_path| on the file system.
   // |src_file_path| can be a hosted document (see limitations below).
   // |dest_file_path| is expected to be of the same type of |src_file_path|
   // (i.e. if |src_file_path| is a file, |dest_file_path| will be created as
   // a file).
   //
   // This method also has the following assumptions/limitations that may be
@@ skipping 162 matching lines @@
                                const FilePath& virtual_dir_path,
                                scoped_ptr<DocumentEntry> entry,
                                const FilePath& file_content_path,
                                GDataCache::FileOperationType cache_operation,
                                const base::Closure& callback) = 0;
 };
 
 }  // namespace gdata
 
 #endif  // CHROME_BROWSER_CHROMEOS_GDATA_GDATA_FILE_SYSTEM_INTERFACE_H_