Refactor unitTable access

Following the review of https://codereview.chromium.org/209443002/ , I've changed the unitTable access method to be a static CSSPrimitiveValue::fromName method.

BUG=

Review URL: https://codereview.chromium.org/209443003

git-svn-id: svn://svn.chromium.org/blink/trunk@169802 bbb929c8-8fbe-4397-9dbb-9b2b20218538