PDF Viewer/Print Preview: Fix zoom buttons touch highlights

A square touch highlight appears on the PDF viewer buttons now that
they have been converted to cr-icon-button. This is likely because
the icon div and iron-icon element are still square, while the
cr-icon-button is circular (styled with border-radius: 50%). Add a
mixin to cr-icon-button to allow the border-radius of the parent
cr-icon-button, the div, and the iron-icon to all be set to the same
value, and use this new mixin for the PDF viewer zoom buttons.

Bug: 964875
Change-Id: I28785d63fd0ee9710dd37a90ac10932d3865a439
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627753
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663253}
2 files changed