[perf] Improve ComputeBordersInternal

This is super minor, but ComputeBordersInternal isn't guarded like
margin/padding. The logical variants of Border*Width create a
PhysicalToLogical<int> each time doing the conversion multiple times.

Just do one conversion at the end.

Change-Id: Ia4b366cb07e99730a13f3fbe55010eb22bb74a30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6496529
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1453598}