don't create zero length intervals

Dashing a pattern without zero-length intervals should
not create them if the end of the on interval coincides
with the beginning of the initial dash offset.

R=reed@google.com
BUG=591993
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1766243004

Review URL: https://codereview.chromium.org/1766243004
2 files changed