Print Preview: Pages and scaling section behavior adjustments

Per UX feedback making Scaling and Pages sections behave the same way:
- scaling custom input is automatically focused when custom is
selected
- pages is automatically populated with the default "1-N" value, if
the field is blurred while empty.

Also addressing attached bug - if user clears a valid input and then
selects "All" pages and then returns to "Custom", their previous valid
input should not be restored.

Bug: 931152
Change-Id: I137969b46f18eee33f5a95bedd82a0dbe61a12b4
Reviewed-on: https://chromium-review.googlesource.com/c/1469291
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632099}
4 files changed