Add oblique as a slant.

Some fonts have italic and oblique in the same family, see

http://lucidafonts.com/fonts/family/lucida-sans
http://www.gust.org.pl/projects/e-foundry/latin-modern

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1921903002

Chromium side change at https://crrev.com/1921503006/ .

Review-Url: https://codereview.chromium.org/1921903002
18 files changed