Simplify -webkit-line-box-contain parsing

This patch reduces code duplication a little in -webkit-line-box-contain parsing.

Review URL: https://codereview.chromium.org/429803004

git-svn-id: svn://svn.chromium.org/blink/trunk@179383 bbb929c8-8fbe-4397-9dbb-9b2b20218538