Remove zoom special casing of SVG when computing border-widths

Ever since border-image-width was implemented in

https://bugs.webkit.org/show_bug.cgi?id=67657

border-widths values haven't been scaled with the zoom level for SVG
elements. Remove the special casing since it doesn't seems to be any
good reason for having it. Firefox doesn't do anything of the sort and
there is no specification support for it.

BUG=377447

Review URL: https://codereview.chromium.org/302643002

git-svn-id: svn://svn.chromium.org/blink/trunk@174960 bbb929c8-8fbe-4397-9dbb-9b2b20218538