Infra: Enable cross-repo tryjobs

Bug: skia:
Change-Id: I560eb00b4b19643346b038822869e5bc77ce1454
Reviewed-on: https://skia-review.googlesource.com/13652
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
6 files changed