add arc circle gap gm

The circle and arc don't line up because
the circle uses conics and arc uses
quadratics.

TBR=reed@google.com
BUG=578885
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1606263002

Review URL: https://codereview.chromium.org/1606263002
1 file changed