A previous patch added clamping for sub-pixel borders to ensure they
did not round to zero size. The change was done in paint code, leaving
the borders as sub-pixel sized in layout. The code prior to that patch
clamped the borders during style conversion but did not account for zoom,
meaning borders disappeared under zoom.

This change removes the clamping in paint and reverts to clamping in the
style conversion code, only this time accounting for zoom when considering
whether to clamp or not. This will prevent borders growing too large as
content is made larger, while preventing sub-pixel borders from disappearing
as content is made smaller.

Design Doc: https://docs.google.com/document/d/1fAYkOFxp2Luh6OOoXxtwOehmvNRGNss58ibQtVXL0Tw/edit?usp=sharing

Bug: 763402, 834489
Change-Id: Iba2ae80b0a981736b93453c1058fd586ba10212c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097514
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750231}