Update dm_flags.json to match dm_flags.py

dm_flags.json verifies that the changes to dm_flags.py made sense. It
was not updated in crrev.com/1537793002, making me confused why my
.py changes seemed to affect WritePixelsTest in .json.

TBR=jvanverth@google.com
BUG=skia:2733
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1534043003

Review URL: https://codereview.chromium.org/1534043003
1 file changed