luci: enable stale tryjob cancellation.

Bug: 959436
Change-Id: I13d8c704a8f8bf3b0d8c66b8cc55a6dc06793d75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032714
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737960}