Add unit test for crbug.com/570232

This is the promised followup to https://codereview.chromium.org/1539553002/ (Add default ctor to SkMask) and would've caught the bug before it got out into the wild.

BUG=570232
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1552303002

Review URL: https://codereview.chromium.org/1552303002