[css-flexbox] Simplify code a bit

No need for these additions/subtractions of border and padding.

R=eae@chromium.org,dgrogan@chromium.org
BUG=470729

Review-Url: https://codereview.chromium.org/2156323002
Cr-Commit-Position: refs/heads/master@{#406151}
2 files changed