[swarming] add debug log to _expire_task_tx
[swarming] add debug log to _expire_task_tx

Adding a debug log to investigate "IndexError: list index out of range _expire_task_tx"
I doubt that 'result_summary.current_task_slice' is old for some reason.

Bug: 1030504
Change-Id: I3d18455ba73580a26bfe22f80d8332b05339f2f4
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1948019
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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