Fix underline position and thickness calculation

Use the underline offset and thickness data from the font when
calculating the desired thickness and position. This depends on the Skia
change https://codereview.chromium.org/271333002 which added the
required fields.

BUG=375454
R=bungeman@chromium.org

Review URL: https://codereview.chromium.org/295923006

git-svn-id: svn://svn.chromium.org/blink/trunk@174431 bbb929c8-8fbe-4397-9dbb-9b2b20218538