CSS: Drop the quirks-mode exception for CSS MIME types.

      This matches Firefox's behavior (though it returns 'transparent' instead of
'rgba(0, 0, 0, 0)', which is a bit annoying).

BUG=419383

Review URL: https://codereview.chromium.org/733993002

git-svn-id: svn://svn.chromium.org/blink/trunk@189669 bbb929c8-8fbe-4397-9dbb-9b2b20218538