Clean up public interface to CSSPropertyParser

This patch makes the CSSPropertyParser public interface static-only.
Currently we construct one to call parseValue once on it, but it makes
more sense to have parseValue as a static function.

BUG=404023

Review URL: https://codereview.chromium.org/515833002

git-svn-id: svn://svn.chromium.org/blink/trunk@181167 bbb929c8-8fbe-4397-9dbb-9b2b20218538