Use plain int for borders widths when possible

Chromium coding style (not overridden by Blink's in this case)
states:

  Do not use unsigned types to mean "this value should never be < 0".

The BorderValue width storage is currently 26 bits wide so it's easily
contained in a plain int.

No functional change expected.

BUG=496033

Review URL: https://codereview.chromium.org/1182053003

git-svn-id: svn://svn.chromium.org/blink/trunk@197111 bbb929c8-8fbe-4397-9dbb-9b2b20218538
5 files changed