Add support for roots.
Change-Id: Ia50abc88f0047238272d698f30ce615b1a8fd486
Reviewed-on: https://go-review.googlesource.com/c/tools/+/671360
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
