[scheduler] Check timestamp field for overflow.
[scheduler] Check timestamp field for overflow.

R=a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org

Change-Id: I8d4c22ac2418e196da7a794547ca229a78c4c89c
Reviewed-on: https://chromium-review.googlesource.com/889702
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go