Test rendering of non-integer border-image-width values

Compare computed non-integer border-image-width values with the
rendering of the respective computed border-width value.

BUG=318081

Review URL: https://codereview.chromium.org/81163002

git-svn-id: svn://svn.chromium.org/blink/trunk@162635 bbb929c8-8fbe-4397-9dbb-9b2b20218538