Print Preview: Only check Chrome OS only policies on Chrome OS

Only use color and duplex setByPolicy on Chrome OS, since these
policies aren't implemented on other platforms.

To make this simpler, move logic for computing whether any setting on
the page is managed out of header.js and into model.js. This is also
more reasonable since model.js is responsible for tracking the settings
and is the only element where settings' |setByPolicy| field is updated.

Change-Id: Ie682c38ffd775cccc4f47c15b66fa85a46761e20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1500785
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637584}
7 files changed