Instead of actually selecting the computed value, add an "Unchanged" option similar to the keyword editor.
<rdar://problem/22338091>
Created attachment 259347 [details] Patch
Comment on attachment 259347 [details] Patch Clearing flags on attachment: 259347 Committed r188628: <http://trac.webkit.org/changeset/188628>
All reviewed patches have been landed. Closing bug.