HTMLSelectElement does not include selected index/indices while saving state

Since index is not part of the FormControl state for select, there is no
way to restore it to proper index if there are duplicate values. This
patch includes the index in the FormControl state for select.

BUG=401506

Review URL: https://codereview.chromium.org/541693003

git-svn-id: svn://svn.chromium.org/blink/trunk@181883 bbb929c8-8fbe-4397-9dbb-9b2b20218538