gdata: Make callback parameters mandatory for TransferFile 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 128 matching lines @@
   // Can be called from UI/IO thread. |callback| is run on the calling thread.
   virtual void GetEntryInfoByResourceId(
       const std::string& resource_id,
       const GetEntryInfoWithFilePathCallback& callback) = 0;
 
   // Initiates transfer of |remote_src_file_path| to |local_dest_file_path|.
   // |remote_src_file_path| is the virtual source path on the gdata file system.
   // |local_dest_file_path| is the destination path on the local file system.
   //
   // Must be called from *UI* thread. |callback| is run on the calling thread.
+  // |callback| must not be null.
   virtual void TransferFileFromRemoteToLocal(
       const FilePath& remote_src_file_path,
       const FilePath& local_dest_file_path,
       const FileOperationCallback& callback) = 0;
 
   // Initiates transfer of |local_src_file_path| to |remote_dest_file_path|.
   // |local_src_file_path| must be a file from the local file system.
   // |remote_dest_file_path| is the virtual destination path within gdata file
   // system.
   //
   // Must be called from *UI* thread. |callback| is run on the calling thread.
+  // |callback| must not be null.
   virtual void TransferFileFromLocalToRemote(
       const FilePath& local_src_file_path,
       const FilePath& remote_dest_file_path,
       const FileOperationCallback& callback) = 0;
 
   // Retrieves a file at the virtual path |file_path| on the gdata file system
   // onto the cache, and mark it dirty. The local path to the cache file is
   // returned to |callback|. After opening the file, both read and write
   // on the file can be done with normal local file operations.
   //
@@ skipping 182 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_