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This fixes a Polymer timing issue where setting the margins in the
margin-control-container was causing the observer in margins-settings
that updated the UI to not fire.

Also moving the margins setter in pages_per_sheet to margins for
consistency.

Bug: 1041378
Change-Id: I83bebfefe8fb306bc332844e5a146271a7ac2b96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000523Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732745}