DM: dump a zero-results .json file right at startup.

This way, if DM runs, the file will be there, even if it crashes before the
first time we'd ordinarily write out the file (currently, any time
the number of tasks remaining to do becomes evenly divisible by 500).

BUG=skia:4839
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1626493002

Review URL: https://codereview.chromium.org/1626493002
1 file changed