Print Preview: Disable custom margins controls when state is not READY.

Like the controls in the sidebar, the custom margins controls should be
disabled if the Print Preview is not ready, there is a validation error
on the page, etc.

Bug: 940386
Change-Id: I2d92c3c13835626b04288c805273ffbcfc3d364b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618641
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662040}
6 files changed