[CSS Shapes] Remove FIXME referencing closed bug

This FIXME is incorrect these days.

This is a merge of http://trac.webkit.org/changeset/167281 by Bem Jones-Bey <bjonesbe@adobe.com>.

NOTRY=true

Review URL: https://codereview.chromium.org/239563002

git-svn-id: svn://svn.chromium.org/blink/trunk@171672 bbb929c8-8fbe-4397-9dbb-9b2b20218538