Selecting OptGroup label does not deselect selected item

OptGroup since it is a label, if selected should not deselect selected item. In updateStateSelected, it is checked if optGroup, no UI changes are triggered.

R=keishi1, tkent
BUG=342189
TEST=On selecting optGroup label, selectedIndex should not be deselected.

Review URL: https://codereview.chromium.org/204433002

git-svn-id: svn://svn.chromium.org/blink/trunk@171470 bbb929c8-8fbe-4397-9dbb-9b2b20218538