Add experimental Swarmbucket bot

Docs here:
https://chromium.googlesource.com/infra/infra/+/master/appengine/cr-buildbucket/doc/swarming.md

NOTRY=true
BUG=skia:4763
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1820323002

Review URL: https://codereview.chromium.org/1820323002
1 file changed