Add download_stats.cc to the directions for modifying download_file_types

It would be nice to have all the changes needed to modify
download_file_types.asciipb in one place, so we don't forget any.

Change-Id: I0a601d28ea6c7ce77cb5e4338746f01cd06ee4de
Reviewed-on: https://chromium-review.googlesource.com/c/1377143
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616515}