[iOS] <select> with <optgroup> may change initial selected option when assisted
https://bugs.webkit.org/show_bug.cgi?id=137261

Patch by Joseph Pecoraro <pecoraro@apple.com> on 2014-10-01
Reviewed by Enrica Casucci.

The initial assignemnt of _singleSelectionIndex was calculating the
item index (ignoring groups) instead of the row index (affected by
groups!). Thus if there was N groups we would select the row N
before the real selection.

Also avoid this entirely if this is a multiple selection picker.
This logic is only needed for single selection with groups.

* UIProcess/ios/forms/WKFormSelectPicker.mm:
(-[WKMultipleSelectPicker initWithView:]):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174148 268f45cc-cd09-0410-ab3c-d52691b4dbfc