Use whole number of seconds in Attribution interop time origin

Otherwise, the adjusted scheduled_report_time field can be off by 1
second due to sub-second values in the time origin and actual report
time.

The change in behavior prior to this was an unintended side effect of
http://crrev.com/c/5317854.

Change-Id: I3308bb517222b083b056e04e755cd55472140405
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5333472


Auto-Submit: Andrew Paseltiner <apaseltiner@chromium.org>
Commit-Queue: Nan Lin <linnan@chromium.org>
Reviewed-by: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267540}