swarming: simplify expiration offset calculation in new_task_to_run
swarming: simplify expiration offset calculation in new_task_to_run

Bug: 1186759
Change-Id: I16e5ebb05af9f8b0736a2623c7c159bbce6a0106
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2784165
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py