Removed -webkit-marquee-increment leftovers.

Only html attribute lengths are now supported remove recognition of keywords
from the CSS property. Gets rid of the custom style builder function by
using generated length conversion method instead.

Review URL: https://codereview.chromium.org/444803002

git-svn-id: svn://svn.chromium.org/blink/trunk@179698 bbb929c8-8fbe-4397-9dbb-9b2b20218538