[Print Preview] Round custom margins values loaded from pref file

The custom margins values used in Print Preview should be integers, but
some users may still have old pref files where they are stored as
doubles. The values are first loaded in the frontend's TS code, and
later sent to the backend, where the C++ code blows up because it
expected ints but got doubles.

Help users in this rare situation avoid the crash by sanitizing the
values in the TS code.

Along the way, consolidate tests that are Chrome OS only.

Bug: 348806045
Change-Id: I35c9b9f7a726cc8a2c362d5bb0a20d56e42a1201
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5943903
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1371785}