Looking at network requets, I see the error in buildset:

buildset:patch/gerrit/undefined/8762/1

"undefined" is supposed to be "skia-review.googlesource.com"

So maybe adding a `buildbucket.config` in `refs/meta/config` for `All-Projects` with:
[host]
  git = "skia.googlesource.com"
  gerrit = "skia-review.googlesource.com"

Should fix this?

This request returns [] on skia host:

https://skia-review.googlesource.com/projects/skia/buildbucket~config

but on chromium host:

https://chromium-review.googlesource.com/projects/infra%2Finfra/buildbucket~config

it returns 

{
  "git_host": "chromium.googlesource.com",
  "gerrit_host": "chromium-review.googlesource.com"
}

So, that's where the error come from.

So, Re#3: yes.

https://chromium.googlesource.com/All-Projects/+/refs/meta/config

Andrii, showed me how to quickly add buildbucket.config and https://skia-review.googlesource.com/projects/skia/buildbucket~config now returns data.

Looks like trybots are showing up: https://skia-review.googlesource.com/c/8500/

Thanks Andrii!