bug=79427.
This checkin adds the following behavior:
On GetValue, if a checkbox or radio button, it returns "CHECKED" or "UNCHECKED".
On SetValue, any string not equal to the empty string causes the checkbox
to be set.
git-svn-id: svn://10.0.0.236/trunk@96927 18797224-902f-48f8-a5cc-f745e15eee43