per beltzner:
"The close button should be shown, with the effect being that it disposes the
tab and immediately opens a new one. Strange behaviour surely, but I think it's
the easiest and most reliably understandable case."
thanks to Simon <zeniko@gmail.com> for the patch.
r=sspitzer, sr=mconnor
git-svn-id: svn://10.0.0.236/trunk@203433 18797224-902f-48f8-a5cc-f745e15eee43