Remove outdated comment in CSSPropertyParser

The comment is outdated since the css values have been
implemented and the referenced bug is closed.

NOTRY=true

Review URL: https://codereview.chromium.org/562743003

git-svn-id: svn://svn.chromium.org/blink/trunk@181778 bbb929c8-8fbe-4397-9dbb-9b2b20218538