Remove CSSValue::Type and CSSValue::cssValueType()

This interface isn't particularly useful and since we no longer expose
it to the web we don't need to keep it around.

Review URL: https://codereview.chromium.org/719993002

git-svn-id: svn://svn.chromium.org/blink/trunk@185253 bbb929c8-8fbe-4397-9dbb-9b2b20218538