For tryjobs, determine the remote branch using GerritAccessor

Bug: 761488
Change-Id: Ieea6028d222af0014ec3a57c3ad7b7eb8af5ec33
Reviewed-on: https://chromium-review.googlesource.com/834995Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>