Print a message when DM finishes

When running from the command line, it is obvious when DM finishes,
since it brings the prompt back up. But in XCode, it just sits
there, so my glance doesn't let me know that it's finished. Make
it more obvious that DM is done.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1650113002

Review URL: https://codereview.chromium.org/1650113002
1 file changed