suite_scheduler: Make trigger_type an enum.

BUG=none
TEST=./generate

Change-Id: Iae475e81bc0bc111128915c8c200af581cc55b38
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2762644
Reviewed-by: Taylor Clark <taylormade@google.com>
Commit-Queue: Brigit Rossbach <brigitr@google.com>
2 files changed