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This code can use TimeDelta::operator%() to express the same thing in a
shorter way.  Add a comment on one less-obvious block.

This relies on the buckets exactly dividing the duration.  Add DCHECKs
and documentation to that end (it is already true in all users).

Bug: none
Change-Id: I67a6269de8fc4a56c716decd5d06008e680f7fec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2328297
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793038}