gdata: Make callback parameters mandatory for TransferFile functions

chrome/browser/chromeos/gdata/gdata_file_system_interface.h

Index: chrome/browser/chromeos/gdata/gdata_file_system_interface.h
diff --git a/chrome/browser/chromeos/gdata/gdata_file_system_interface.h b/chrome/browser/chromeos/gdata/gdata_file_system_interface.h
index 339f96b02518a3d188b474d61bb2aea71dcec4c9..8d680d085d7fbac7103bd6d853f1c917c2cfa52b 100644
--- a/chrome/browser/chromeos/gdata/gdata_file_system_interface.h
+++ b/chrome/browser/chromeos/gdata/gdata_file_system_interface.h
@@ -146,6 +146,7 @@ class GDataFileSystemInterface {
   // |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,
@@ -157,6 +158,7 @@ class GDataFileSystemInterface {
   // 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,