Define DateTimeEditBuilder step using base::TimeDelta

Convert to use the base::Time helpers to generate a base::TimeDelta of
the right length and then convert to milliseconds in CreateStep().

This eliminates a bunch of uses of the kMs... constants from
platform/wtf/date_math.h.

Bug: 446602676
Change-Id: I10b4698e1be44e21d752c40ed6c15df757f8e3ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6994196
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1522539}