Document timestamp caveat in FileSystem API move operation.

We don't plan to fix the bug, because the FileSystem API is in
maintenance mode. We document the bug as a heads-up to anyone (most
likely us in the future) who might want to reuse the code.

BUG=171284

Review-Url: https://codereview.chromium.org/2390323003
Cr-Commit-Position: refs/heads/master@{#423034}

storage/browser/fileapi/file_system_operation.h

@@ -297,6 +297,8 @@ class FileSystemOperation {
   //   CopyInForeignFile and CreateDirectory on dest filesystem
   //   for cross-filesystem case.
   //
+  // TODO(crbug.com/171284): Restore directory timestamps after the Move
+  //                         operation.
   virtual void Move(const FileSystemURL& src_path,
                     const FileSystemURL& dest_path,
                     CopyOrMoveOption option,