Fix table-cell-offset-width to properly use the Ahem font.

Previously this test did not actually use Ahem which caused cross-
platform failures.

TBR=eseidel
BUG=290399

Review URL: https://codereview.chromium.org/25037012

git-svn-id: svn://svn.chromium.org/blink/trunk@158492 bbb929c8-8fbe-4397-9dbb-9b2b20218538