Print Preview: Add validity check for kSettingPagesPerSheet.

It was not checked because pagesPerSheet was only added to the new
print preview UI, adding the validity check caused coredump.  Now
since the old print preview UI is gone, we can add the check.

Bug: 842000
Change-Id: I413be9f89c7baca135275bd27fc228e23d0f202e
Reviewed-on: https://chromium-review.googlesource.com/c/1381133
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Shirleen Lou <xlou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617334}